From the Flour cookbook, Joanne Chang
1/2 recipe for Brioche dough
1 recipe for Pastry cream
4 oz (114 grams) bittersweet chocolate chips
Line a baking sheet with parchment paper.
Roll out the dough on a floured work surface into a 20×10″ rectangle. It feels like Play-Doh. With the 20″ side facing you, spread the pastry cream over the entire rectangle surface. Then sprinkle the bottom half of the rectangle with the chopped chocolate. Fold the top half of the rectangle over the bottom half and press down to smoosh the halves together.
Cut the filled dough into 10 pieces (about 2 inches wide). Transfer the pieces to the baking sheet, cover with plastic wrap, and allow them to proof for 2 hours in a warm place (Or freeze them wrapped in plastic wrap for up to a week: thaw the wrapped pastries in the fridge overnight and then proceed with the proofing step). The dough will get puffy.
Preheat the oven to 350 degrees.
Whisk the egg in a small bowl and brush the tops of the pastries with the beaten egg. Bake for 35-45 minutes or until golden brown. Let them cool on a the baking sheet on a wire rack for 20-30 minutes.