Wash the blueberries in a colander under running water. Place them in a saucepan and add sugar and lemon juice and zest. Bring the blueberries to a boil slowly while stirring continuously with a spoon. Stir from the sides towards the center to avoid burning.  Let the berries boil for about 3 minutes, but not longer. We want them whole. Dissolve pudding powder or cornstarch and vanilla with 5 tablespoons of cold water and add to the berries. Stir with your spoon until the color becomes red. Let bubble up once. Pour the berry mixture in a bowl and wait a bit until it has cooled down. Then pour it into your baking pan on top of the pastry. Make sure you pour the filling evenly so that the mixture fills all the way to the edges of the pastry.

Use a flour sifter and sift the flour with the baking powder together into a bowl. Add the sugar, vanilla sugar, and salt. Mix everything well. Make a hole in the center of the flour and add the egg. Cut the butter into cubes and spread on top. With your hands or using a hand mixer with the kneading hooks, knead all the ingredients together until you have a  smooth dough. The dough is ready when it is no longer sticky.

Line the springform with parchment paper. Use a bit more than the half of the Pastry dough to spread it on the bottom of the springform by hand, press some up the sides. Divide the rest of the dough in two. This is for the outer crust. Form two rolls and place them around the inside of the springform pan. Using your fingers, press the rolls against the sides of the springform (about 2 inches high) and connect it to the bottom part of the dough. Make sure all the dough is sealed at the edges. Use a fork to make a few small holes in the dough. Pour filling on top.

Add flour, sugar, vanilla sugar and butter in a bowl and knead with the dough hooks of your handheld mixer. When combined, rub the dough between your hands. The crumbles should fall back into the bowl. Make sure that all the butter is worked well into the crumbs. If your crumbles are too dry, add a little more butter. If the crumbles are too moist, add some flour. Spread the crumbles evenly on top of your cake.

Preheat your oven to 182°C (360°F) and place the cake on the middle rack. Bake for about 40-45 minutes. I always move any cake with a crumble topping one rack up for the last five minutes to give the crumbles a deeper golden color. Let the cake cool down in the spring-form pan, before you take it out. Sprinkle some powdered sugar on top. Carefully move the cake with the help of a cake lifter onto the serving plate. Serve with whipped cream.