In a big bowl, combine the flour, salt, sugar, and butter with your fingertips until remains only small peas.
In a small bowl, lightly whisk the egg with a fork. Add to the dough and mix until forms a rough ball (don't knead it). Rest in the fridge for about 15 min.
Preheat oven to 180 C temp. Grease the edges of 22 cm round cake pan, line the bottom with parchment paper.
Dust a surface with flour and roll out the dough to circle. Transfer into a pan. Form with hands about 3 cm edges. Prick the base with a fork, cover a parchment paper and fill with uncooked beans, lentils or rice. Bake about 15 min. Remove the beans and paper.
Meanwhile, in a bowl, lightly whisk egg and sugar. Then add other filling ingredients (except bilberries) and mix just until combined.
Add 250 g bilberries on the bottom. Pour the filling. Drop 50 g bilberries on the cake top.
Bake about 30-35 min. Remove from a pan when completely cool.