Base:

Cup of Yum
1. Mix the flour with the salt, baking powder, and powdered sugar. Cut the butter into cubes and add to the flour mixture. Rub with your fingertips until you get a sandy dough.
2. Lightly whisk the egg yolks, add 2-3 tablespoons of cold water (from the refrigerator), mix, then add to the dough and bring the dough together gently until smooth. If the dough is not bound at this point, add another tablespoon of cold water.
3. Wrap the dough in cling film and let it chill in the refrigerator for an hour. Then remove from the fridge, cut off a third and put it back in the fridge.
4. Place the remaining dough on a well-floured worktop. Roll it out into a rectangular sheet, slightly larger than the size of the tray (I used a 25/35 cm or 10/14 inch tray). It should be 0.5mm (0.2 inch) thick.
5. Preheat the oven to 180 °C (356 °F). Grease a 25/35cm (10/14 inch) baking tray with butter.
6. Place the sheet of dough in the baking tray, leveling it out. Gently roll up the sides of the dough to cover the sides of the tray. Poke it a fork. Bake for 15 minutes, without the filling.

Filling:

1. Whisk the egg whites lightly with the sugar and a pinch of salt. They don't need to make a firm foam, whisk it just enough to break the whites. Add in the yogurt and mix.
2. Pour the mixture over the base in the pan and place the fruit on top: blueberries, raspberries, and currants.
3. Remove the third of the dough from the refrigerator. Tear off small pieces and sprinkle them over the fruit. Put back in the oven (also at 180 °C/ 356 °F) and bake for 20-25 minutes, until the crispy layer on top is golden brown.
4. When ready, transfer to a cooling rack. Decorate with melted white chocolate. It can be served with vanilla ice cream.

It's delicious!