Bring the fat, 1/4 litre water and salt to the boil in a saucepan. Add the flour at once and stir until the dough separates as a lump from the bottom of the pot. Remove the pot from the heat and stir in the eggs bit by bit.
Leave to rest for about 1/2 hour. Pour dough into a piping bag with a large star-shaped spout and squirt seven rings and V's each onto a baking tray lined with baking paper. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 40 minutes.
In the meantime, sort and wash the blueberries. Whip the cream until stiff. Mix quark, sugar, vanilla sugar, lemon juice and zest. Fold in cream and blueberries. Finely chop the pistachios.
Cut the still warm pastry pieces horizontally and let them cool down. Spread the lower halves with blueberry quark, place the upper halves on top. Sprinkle with the pistachios and dust with icing sugar.
Arrange on a plate with some mint leaves. Makes 14 pieces.