Kremowka (pronounced kreh-MOOV-kah) is a Polish cream cake that sandwiches vanilla pastry cream between two layers of puff pastry. Martha made this recipe on episode 611 of Martha Bakes.
Preheat oven to 400 degrees with racks in upper and lower third position and line two baking sheets with nonstick baking mats. Divide puff pastry into two equal pieces. Sprinkle work surface with granulated sugar and roll out each piece of pastry to a 9-inch-by-13-inch rectangle. Transfer to prepared baking sheets and bake, rotating halfway, until puffed and deep golden brown, about 35 minutes. Cool completely on a wire rack.
Whisk together 3/4 cup granulated sugar, cornstarch, and salt in a medium saucepan. Whisk together milk and egg yolks in a glass measuring cup. Add milk mixture to the saucepan, along with 3 tablespoons butter. Cook over medium heat until mixture comes to a boil. Let boil 1 minute. Remove pastry cream from heat.
Strain pastry cream through a fine-mesh sieve into a bowl. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or up to 2 days.
Place remaining 3 sticks of butter in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until smooth. Reduce speed to low and add vanilla bean seeds and cooled pastry cream, 1/2 cup at a time. Increase speed to medium-high and beat until light and fluffy, about 2 minutes.
Place one rectangle of puff pastry on a cutting board and spread with pastry cream mixture. Top with second puff pastry rectangle and press gently to adhere. Loosely cover and refrigerate to set, at least 1 hour or up to 8 hours. To serve, slice with a serrated knife into 3-inch squares. Dust with confectioners' sugar.