Cut butter into pieces. Mix the flour, butter, icing sugar and egg yolks first with the dough hooks of the hand mixer, then with the hammers to a smooth dough. Wrap the dough in foil and chill for about 30 minutes. Sort the blackberries. Mix sour cream, sauce powder, sugar and eggs. Knead the dough again and roll it out on a floured work surface (28 cm Ø).
Line a greased tart tin (top approx. 26 cm Ø, bottom 24 cm Ø) with the dough, press on the edges and cut off any protruding edges. Pour the mixture into the mould, then spread the berries on top. Bake the tart in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes. Sprinkle the flaked almonds onto the tart about 5 minutes before the end of the cooking time and bake with it. Remove the cake from the oven and let it cool down on a cake rack
1 1/2 hours waiting time