Mix flour, sugar, yeast, and salt in a bowl. In the center, press a hole, and pour in egg and 2 egg yolks. Pour flour mixture over the top. Warm up the milk to lukewarm and pour it over the flour mixture. Melt butter and add it to the bowl. Use a mixer with kneading hook and mix until dough is no longer sticky and it starts to bubble. Cover with a kitchen towel and let dough rise about an hour.
Prepare crumb mixture by mixing flour, sugar and vanilla together. Mix in small pieces of butter (cold but not soft) until it resembles a crumb mixture. Don't overwork it or it will turn into a dough.
Preheat oven to 375.
Cover a large rimmed baking sheet with parchment paper. Roll the dough out or press out using your floured hands. Don't overwork it. Make sure it's even. Cover with dough towel and let rise for 30 minutes. Mix egg yolk and 2 Tbs milk and brush onto the dough. Spread crumb mixture over cake.
Bake the cake in the preheated oven on center rack for approximately 20-25 minutes. Remove the cake, and sift with powdered sugar if desired.