In a large bowl, add flour, baking powder and salt together and give it a rough mix. Add the softened butter and combine it using your finger tips until it resembles grains of sand.
Add the water and 25g castor sugar to the egg yolks and give it a quick mix. Add the egg yolk mixture to the flour-butter mixture and using a fork, mix until a dough forms.
Lightly dust a clean working surface with flour and roll out the dough. You want it to be about 3-5mm thick. Use a cookie cutter to cut out the circles. The cookie cutter should be slightly bigger than the muffin hole's size.
Evenly press the pastry in each muffin hole, ensuring it's pressed firmly around the edge and the base of the tin - I use the base of the handle on my spatula to secure the pastry in the mold.