Make a batter from the butter, 100 g sugar, 1 egg, the flour and the baking powder and place in a flat, greased cake tin or a 28-inch tart tin.
In a bowl, mix Kermaviili (the Finnish product) or Skyr (possibly more readily available) or quark), 50 g sugar, an egg, vanilla sugar and blueberries and pour onto the batter in the cake pan.
Bake the cake in a preheated oven at 200 ° C for about 30 to 40 minutes.