Sugar Buns

Line a large rimmed baking sheet with parchment paper; set aside.
On a lightly floured work surface, roll out dough to a 15-by-12-inch rectangle, about 3/8 inch thick.
Using a ruler and pizza wheel or pastry cutter, cut dough into twenty 3-inch squares.
Transfer the Pastry Cream to a pastry bag fitted with a 1/2-inch plain tip (such as an Ateco #806).
Pipe 1 tablespoon into the center of each square.
Brush the edges of each square with the beaten egg.
Gather the edges around Pastry Cream, pinching the seams together, making sure the bun is completely sealed.
On a piece of parchment paper, gently roll each bun with floured hands, keeping it seam side down, to form a compact ball.
Repeat with remaining dough.
Place shaped balls, smooth side up, on the prepared sheet, about 3 inches apart.
Cover with plastic wrap, and let rise in a warm spot until doubled in size, about 45 minutes.
Meanwhile, preheat the oven to 400F.
Brush gently with the beaten egg.
Bake, rotating sheet halfway through, until buns are golden brown, 25 to 30 minutes.
Transfer to a wire rack to cool completely.
Brush the butter over the top of the buns.
Place sugar in a wide bowl; dip buttered side of buns into sugar to coat.
Serve at room temperature.
These buns are best eaten the day they are made.
A dollop of Pastry Cream is piped into the center of each dough square.
The corners of the dough are pinched together to seal in the cream.
The filled dough balls are rolled gently back and forth to seal the seam.