Thüringer Mohnkuchen (Thuringian Poppy Seed Sheet Cake)

Instructions

1. For the dough, mix the all-purpose flour, instant yeast, 50 g granulated sugar and salt in a bowl. Add the lukewarm whole milk, the cooled melted butter and the egg and knead by hand or with dough hooks until smooth, soft and slightly sticky, about 5 minutes. Cover and let rise in a warm place until doubled, about 1 hour.
2. While the dough rises, make the custard topping: whisk 3 tbsp of the 250 ml milk with the cornstarch until smooth. Bring the rest of that milk, the 50 g sugar and the 30 g butter to a boil, whisk in the cornstarch mixture and boil, whisking, until thick, about 1 minute. Stir in the vanilla extract, press plastic wrap on the surface and cool completely, about 1 hour.
3. For the poppy seed filling, bring the 350 ml milk, 75 g butter and 100 g sugar to a simmer. Whisk in the semolina, ground poppy seed and salt and cook over low heat, stirring constantly, until thick and spreadable, about 5 minutes. Stir in the raisins and chopped blanched almonds and let cool until lukewarm, about 30 minutes.
4. Grease the baking pan with butter. On a lightly floured surface, roll the risen dough into a 9 x 13-inch (23 x 33 cm) rectangle and press it into the pan, pushing it slightly up the sides.
5. Spread the lukewarm poppy seed filling evenly over the dough and let it stand 20 minutes while you preheat the oven to 350°F (175°C).
6. Whisk the 2 egg yolks into the cooled custard until smooth. Whip the 2 egg whites to stiff peaks and fold them gently into the custard. Spread the custard over the poppy seed layer right to the edges.
7. Bake on the middle rack until the top is set and pale golden and the edge of the base is browned, 40 to 45 minutes. Cover loosely with parchment if the top colors too quickly.
8. Cool completely in the pan on a rack, about 1 hour, then dust with the powdered sugar and cut into 15 pieces.