Angel food sponge cake with egg whites

In a small bowl, combine warm water and yeast. Let stand until foamy, about 5 minutes.

Meanwhile, in a large bowl, combine the all-purpose flour, whole-wheat flour, baking powder, and baking soda. Add the chopped butter to the dry ingredients and mix until the mixture resembles coarse flour.

Place the dough on a lightly floured surface and knead it lightly.

Form the dough into a rectangle. Cut it into 4 pieces and stack them on top of each other. Roll it out. Repeat the process.
Line a baking sheet with a silicone mat or parchment paper. Then cut the dough pieces into squares and place them on the baking sheet.

Cover loosely with foil or a clean kitchen towel and leave in a warm place for about 30 minutes.

Bake the sponge cake in a preheated oven at 180°C for 10-15 minutes until beautifully golden brown.

Transfer to a wire rack and let cool.