In a medium saucepan, whisk together ½ cup cornstarch, 1 cup sugar, and a pinch of salt. Mix 2 cups of milk and 4 egg yolks, incorporate gradually. Cook over medium heat for 1-2 minutes until thickened. Stir in 4 tablespoons unsalted butter and 1 teaspoon vanilla extract, chill for at least 2 hours.

Grate 1 cup of cold unsalted butter into 2 cups all-purpose flour and a pinch of salt. In another bowl, mix ½ cup sour cream, 1 tablespoon white vinegar, 1 egg, and 1 teaspoon vanilla extract. Add this mixture to the flour and butter, mixing with a fork until a dough forms. Chill for 30 minutes to 1 hour.

Preheat your oven to 430°F (220°C). Divide the chilled dough into 9-10 pieces, roll each into a 10-inch circle, and bake for 6-8 minutes. Allow to cool completely on wire racks.

Spread pastry cream over the first layer of puff pastry. Repeat until all layers are stacked, reserving one for crumbling on top. Frost the cake with any remaining pastry cream.

Cover the assembled cake with plastic wrap and refrigerate overnight to allow flavors to meld. Sprinkle reserved crumbled pastry on top before serving.