Butterfly Pea Flower Pudding — Thai blue-and-white coconut cups

i. Origin & history

Khanom thuai means 'cup dessert': thuai is a small cup or bowl, and the puddings are traditionally steamed in tiny porcelain saucers called thuai talai. It is one of the oldest and most common Thai sweets, sold at markets throughout the country and made at home for family gatherings and temple festivals. The classic version uses pandan for a green bottom layer or palm sugar for a brown one; the blue butterfly pea version is equally traditional in many households and has become especially popular in recent years.

Butterfly pea (Clitoria ternatea), called anchan in Thai, is a climbing vine whose deep blue flowers have been used for centuries in Southeast Asia as a natural food colouring and herbal tea. In Thailand the flowers colour sticky rice, noodles, drinks and sweets, and nam anchan — butterfly pea tea, often with lime — is a common drink. In Malaysia the same flower colours the blue rice nasi kerabu and the blue streaks in Nyonya kuih.

The two-layer structure of khanom thuai is shared by many Thai sweets. The bottom layer, called tua (body), is sweet; the top, hua kathi (coconut head), is a salted coconut cream thickened with rice flour. The same salted coconut top is used on khanom krok and coconut sticky rice, and the balance of sweet and salty is a hallmark of Thai desserts.

Similar cup puddings are found across the region: Malaysian kuih talam uses the same two layers in a tray, and Indonesian kue talam is a close relative. The blue-and-white colour scheme has become a favourite of modern Thai cafés, which serve khanom thuai alongside butterfly pea drinks.

ii. Why the blue changes colour

Butterfly pea's blue comes from anthocyanins — specifically a group called ternatins, which are unusually stable blue pigments. Like all anthocyanins, they are sensitive to pH: at neutral pH they are deep blue, in acidic conditions they shift to purple and pink, and in alkaline conditions they turn greenish. That is why a squeeze of lime turns butterfly pea tea violet, a trick used in colour-changing drinks.

For the pudding, you want the batter neutral, so it stays blue. Coconut milk is slightly acidic but not enough to change the colour much. Avoid adding lime or lemon to the blue layer, and use plain water rather than tap water if yours is very hard, as alkaline water can tint the blue towards green.

The texture comes from rice flour and a little tapioca or arrowroot starch. The bottom layer is thin and pourable and sets in the steamer into a soft, smooth pudding; the top layer, with more coconut and less water, sets into a silky, creamy cap. The cups are heated in the steamer first, so the batter begins to set immediately on contact, giving the characteristic slight dip in the centre.

The bottom layer must be set before the top is added, or the layers will mix. Steaming the bottom for about 8–10 minutes until it is just firm at the surface, then adding the coconut and steaming again, gives two distinct layers that bond together.

iii. Ingredients

Makes 12 servings · scroll the side panel to adjust

• 30 dried butterfly pea flowers
• 400 ml hot water
• 100 g rice flour (blue layer)
• 20 g tapioca starch or arrowroot
• 150 g sugar
• 100 ml coconut milk (blue layer)
• 400 ml thick coconut milk (white layer)
• 40 g rice flour (white layer)
• ¾ tsp salt (white layer)
• 1 tsp sugar (white layer)

Dried butterfly pea flowers are sold in Asian shops and online, often as tea; fresh flowers work too (use about twice as many). Use rice flour, not glutinous rice flour, which would make the pudding sticky and chewy. Tapioca starch or arrowroot gives a smoother set. Use full-fat coconut milk for the top layer; the creamy richness is essential. Small ceramic cups, silicone moulds or a mini muffin tin all work.

iv. Method

1. Steep the butterfly pea flowers in the hot water for 10 minutes until deep blue, then strain, pressing the flowers. You need about 350 ml blue water; top up with water if needed. Cool.
2. For the blue layer, whisk the rice flour, tapioca starch and sugar with the blue water and 100 ml coconut milk until smooth. Strain.
3. For the white layer, whisk the thick coconut milk, 40 g rice flour, salt and 1 tsp sugar until smooth. Strain.
4. Set up a steamer with water at a full boil, wrapping the lid in a cloth. Arrange 12 small cups (about 50 ml each) in the steamer and heat for 5 minutes.
5. Stir the blue batter and fill each hot cup about two-thirds full. Cover and steam for 8–10 minutes, until the surface is set and no longer liquid.
6. Stir the white batter and spoon it gently over each blue layer to fill the cups. Steam for 8–10 minutes more, until the white layer is set and glossy.
7. Remove the cups and cool completely, at least 1 hour. Loosen the edges with a small knife or spoon and turn out, or serve in the cups.

v. Tips & common mistakes

• Heat the cups first. Hot cups set the batter immediately and give the classic shape.
• Set the bottom before adding the top. Otherwise the blue and white layers blur together.
• Keep acid away from the blue layer. Lime or lemon turns butterfly pea purple.
• Stir the batters before pouring. Rice flour settles quickly and makes uneven layers.
• Wrap the lid. Condensation dripping onto the cups pits the surface and dilutes the coconut.
• Season the top well. The white layer should taste distinctly salty to balance the sweet base.

vi. Variations

Khanom thuai bai toey uses pandan juice for a green base, and the palm-sugar version is brown. Three-colour versions use blue, green and white. Some cooks add a few drops of lime to half the blue batter to create a purple layer, an effect popular in Bangkok cafés.

For other Thai two-layer and coconut desserts, see khanom chan, khanom krok and sankaya. Butterfly pea also colours Malaysian seri muka in some versions, and the coconut pudding coconut jelly is another cool, layered treat.

vii. Serving & storage

Butterfly pea flower pudding is served at room temperature, in the cups or turned out on a plate, as an afternoon snack or dessert. It is often part of a selection of small Thai sweets, served with tea or butterfly pea tea.

It is best on the day it is made, kept covered at room temperature. In hot weather it can be refrigerated for up to 2 days, but the rice flour firms in the cold; bring to room temperature or steam briefly before serving. It does not freeze well.

ix. More from Thai

• Khanom Chan — nine-layer cake
• Khanom Krok — coconut-rice pancakes
• Sankaya — Thai coconut custard
• Tub Tim Krob — 'crisp rubies' in coconut milk
• Coconut Rice Cake — Thai coconut sticky rice
• Seri Muka — Malaysian rice and custard