Cream the butter and sugar together until smooth.
Add the egg yolk and mix well.
Sift in cake flour, baking powder, and salt, then mix to form a dough.
Finely chop the white chocolate and add it to the dough.
Shape the dough into small balls and place them on a baking pan.
Bake in a preheated oven at 180°C (356°F) for 12–15 minutes.