While the dough is proofing, make the custard. In a medium saucepan, add the cream, sugar, cornstarch, and egg yolk. Whisk together until combined. Turn on the heat to medium and heat the custard until it starts to thicken and bubble - about 3 - 5 minutes.
Removed the custard from the heat and stir in the vanilla extract and butter until incorporated.
Strain the custard through a fine mesh strainer set over a bowl. Cover the custard with a piece of plastic wrap directly touching the custard - this prevents a skin from forming. Let cool and sit at room temperature until ready to use.
Shaping the Dough - After the dough has doubled in size, cut into 4 equal pieces. Shape each piece into a tight ball by gathering the edges and pinching together to seal on the bottom of the ball. Gently form into a ball with your hands, and place the seam side down on the prepared baking sheet. Loosely cover the baking sheet with a piece of plastic wrap and allow the dough to proof and rise again, for another 30 minutes.
Preheat the oven to 350F degrees and adjust the oven rack to the middle position.
After Second Rise - Using your fingers or the back of a spoon, gently press down on the center of each dough ball to form a well to hold the custard. Leave yourself enough of an edge to make sure the custard stays inside the pies.
Mix a splash of water with the reserved egg white to make the egg wash. Brush the edges of the dough with the egg wash. Optional - Sprinkle Turbinado sugar onto the edges of the dough.
Fill the wells of the dough with the custard, then top with peach slices. Bake for 18 - 20 minutes, or until the dough is golden brown and the custard has set.
Transfer the mini pies to a wire cooling rack to cool completely and sprinkle with ground cinnamon (optional). The Kuchen are best enjoyed the day they are baked. Enjoy!