Whisk flour and water together in a saucepan over a medium heat and keep whisking until you can see lines in the mixture; try not to overcook. Set aside and let cool to room temperature.
For the topping:
Mix dry ingredients together (flour, cornflour, custard powder, bicarb, baking powder and salt) in one bowl.
In another bowl beat butter and sugar until light and fluffy. Add in the 2 egg yolks and whisk. Combine the two mixtures and form a dough. Wrap in cling film and chill in the fridge until firm.
For the buns:
Heat the milk in a saucepan until it is lukewarm, add in the yeast and 2 teaspoons of sugar and let stand until yeast is bubbly.
Mix the remaining sugar, flours, custard powder, salt in a mixing bowl. Add in the yeast mixture, water roux and an egg.
Mix all the ingredients until it forms a soft, sticky dough. Alternatively, you may pour everything into a breadmachine and put it on dough mode.
Add in the butter and mix, continue beating the dough until the dough is smooth and elastic. Put the kneaded dough in a lightly oiled bowl and cover with cling film or a clean tea towel. Proof for 30 minutes or until the dough doubled in size.
Prepare baking trays with greaseproof paper. You may wish to grease the paper, I find it easier to remove the bun when it's done.
When the dough has doubled in size (or when the breadmaker beeps you), divide the dough into 20 pieces, roll them into 20 balls. Cover with cling film and proof again for 30 minutes and the buns are doubled in size.
Preheat the oven to 180 C / Gas 4.
When the buns are ready. Take the topping from the fridge, divide it into 20 pieces. Flatten each and form a circle, place it on the bun. Beat an egg and brush the top of the buns.
Bake for around 20 minutes, until they have a good brown colour. Allow to cool completely on a wire rack before eating.
You may divide the dough into 16 to 25 balls, this depends on whether you want a standard size buns or mini ones. Minis are great for sharing!