In a large bowl combine dry ingredients (flour, sugar, cocoa powder, and salt.) Cut in the cold butter with a pastry cutter into the flour mixture until the butter is in small pieces, about the size of peas.
Combine cold milk and egg. Then add to flour mixture. Stir until loose pastry dough. Turn out onto a lightly floured surface and work it into a ball. Wrap in piece of plastic wrap and chill for about 30 minutes. (While the chocolate dough is chilling, make chocolate filling.)
After dough has chilled, roll out your chocolate pie dough into about a 16 x 18 rectangle on a floured work surface with a floured rolling pin.
Cut chocolate dough in half with a bench scraper or sharp knife. Next cut strips about 3 inches wide for your pop tarts.
Put a rounded Tablespoon of the cooled chocolate filling on each bottom half of pop tart and and smooth down, leaving room to seal all the edges.
Brush all edges with egg wash to seal the edges. Fold over top of pop tart onto filling. Press edges down and then use the tines of a fork to crimp the edges. Place on a baking sheet lined with parchment paper or silicone mat. Bake in 400 degree preheated oven for 15 minutes.
Add chocolate frosting and sprinkle chocolate sprinkles on top of the pop tart. My taste testers like them when they are room temperature. So let them cool and enjoy. Store in an airtight container.