Combine the coconut milk, pandan juice, and pandan paste in the heatproof pot.

Bring the pandan coconut mixture to a boil while stirring continuously, then let it cool until warm.

Stir the wheat flour, 25 g caster sugar, instant yeast, and warm water in a mixing bowl.

Cover the starter with plastic wrap and rest it for 15 min.

Beat the egg yolks, whole eggs, and 200 g caster sugar for about 10 min until pale, thick, and ribboning.

Sift the tapioca flour and salt into the egg mixture.

Mix the batter only until the tapioca flour is evenly incorporated.

Add the rested yeast starter and mix until smooth.

Add the warm pandan coconut mixture and mix until fully blended.

Replace the mixer with a spiral whisk and slap or whisk the batter for 5 min.

Cover the batter with a kitchen towel and ferment for 120 to 180 min, depending on room temperature.

Brush every inside surface of the cake pan with neutral cooking oil.

Place the clean sand in the large heatproof pan and nest the oiled cake pan in the sand.

Preheat the sand and cake pan over the stovetop for 10 to 15 min until the pan is very hot.

Preheat the oven to 170°C / 340°F using top heat only.

Stir the fermented batter gently and fold in the melted butter.

Pour the batter into the fully heated cake pan.

Cook the cake over the sand bath for about 75 min until set and honeycombed.

Transfer the cake to the oven and brown the top for about 5 min.

Insert a skewer into the center; remove the cake when the skewer comes out clean.

Cool the cake completely in the pan before slicing and serving.