Mix part of the flour, part of the cornstarch, and baking powder in a bowl. Grease a round springform pan, dust with flour, and preheat oven to 200°C/390°F.
Beat eggs, part of the sugar, and salt for approx. 3 – 4 min. until fluffy and airy. Add vegetable oil, part of the milk, and vanilla sugar, and combine well. Carefully fold in flour mixture and cocoa powder. Transfer batter to springform pan and bake in the oven for approx. 20 min. at 200°C/390°F.
Whisk part of the cornstarch into part of the milk.
Heat up remaining milk, part of the butter (cut into pieces) and sugar, poppy seeds, and semolina in a saucepan. Stir until combined. Pour the milk-starch mixture into the poppy seed mixture. Continue to stir for approx. 3 min., or until thickened.
Remove the half-baked cake from the oven and spread the poppy seed mixture on top. Mix remaining flour, sugar, and butter until crumbly. Spread the crumbs on top of the poppy seed mixture and bake for approx. 20 min. at 200°C/390°F until the crumbs are golden. Garnish with confectioner’s sugar if desired.