First things first, pour the warm water into a small bowl, sprinkle over the yeast and sugar and stir together. Leave to stand for 10 minutes until foaming.
In a large bowl or freestanding mixer, mix together the flour and a pinch of salt. Mix the milk, melted butter, Vanilla Extract and vinegar into the yeast mixture and then pour into the flour, finally add a tsp of Red Food Colour Gel to the mixture. Mix to combine and then knead together using either your hand or a dough hook attachment in a freestanding mixer until your dough is smooth, about 10 minutes.
Pour into a bowl greased with oil and cover, leave until doubled in size for about 1½ hours.
Whilst your dough is proving it’s time to make the custard filling, pop the egg yolks, caster sugar and corn flour in a bowl and whisk together to make a smooth paste.
Pour the milk and Vanilla Extract into a saucepan and pop over a medium heat stirring until simmering. Pour half the hot milk mixture into the egg yolk mixture and whisk to combine then pour back into the remaining milk mixture. Pop the pan back over a medium heat and continuously whisk until the mixture begins to bubble. As it bubbles it’ll thicken, continue to whisk until a think consistency and then remove from the heat, make sure your custard has thickened and boiled before removing from the heat as this will help it set.
Pour into a bowl and cover with cling film, ensuring the cling film touches the surface of the custard as this will prevent a skin from forming on the custard. Leave to in the fridge.
Once your dough has doubled in size, tip it onto a lightly floured surface and punch it down, flattening it down to a rectangle. Mix the Baking Powder with 15ml water and brush over the surface of the dough. Knead your dough for 5 minutes.
Divide your dough into 10 equal pieces. One piece at a time flatten it in your hand and add a spoonful of custard into the centre, wrap the dough around the custard and pinch into place, this is the base of the bun. Roll the bun on the surface to smooth out the base. Place into a square of parchment paper, cover and leave to rest for 30-45 minutes. Repeat to shape each bun.
Once your bao buns have rested you are ready to steam them, add some water to the base of your steamer, and bring to the boil. Wrap your pan lid in a tea towel to prevent condensation from dripping onto your buns. Add a 1 or 2 buns to your steamer, leaving room in between them and steam for about 7 minutes, do not uncover your buns as you steam them. Remove from the steamer using tongs and then steam the next batch until all your buns are cooked.
Tuck in and enjoy whilst warm or leave to cool. These are best eaten fresh on the day they are made.