Instructions

1. Whisk together dry ingredients (flour, baking powder and salt) then set aside.
2. Beat softened butter with sugar and vanilla for 2 minutes, then add one by one egg yolk, mixing well after each addition.
3. Add dry ingredients and mix until evenly combined. The mixture will look sandy or crumbly. Knead it with your hands to make the compact dough.  Divide the dough into two equal parts. Wrap each half with plastic foil or place in plastic bags and place one in the fridge and the other in the freezer for 30 minutes.
4. Preheat the oven to 350F. Spray 8 x 8 inches square pan with baking spray and line with baking paper leaving large overhang the sides.
5. Take the dough from the fridge and roll it out on a lightly floured surface in an 8 x 8 inches square. Transfer it and fit in the bottom of the pan. The dough will be thin and if it brakes just pinch it with your fingers to seal the cracks when place it in the pan.
6. To make the filling beat egg whites until stiff peaks form and set aside.
7. In a large bowl mix together beat lemon yogurt, sour cream vanilla and sugar. Mix in flour.
8. Finally, gradually add beaten egg whites and whisk it in the mixture. Pour the mixture over the crust and smooth the top. Take the dough from the freezer and using a coarse grater, grate it over the filling.
9. Bake 35-40 minutes or until the toothpick inserted in the center comes out clean. Tent the top with aluminum foil if it starts browning too much. It will puff and slightly crack when it’s done, but it will sink as it cools.
10. Lift from the pan and transfer on cutting board. Slice and serve. Sprinkle generously with powdered sugar before serving.