To make the chocolate fudge filling:
 In a small saucepan, stir together the cocoa powder, milk, and 3/4 cup chocolate chips.

Set over medium heat and cook, stirring, for 5-8 minutes, until the chocolate is melted and the sauce has thickened slightly.

Remove from heat, stir in the remaining 3/4 cup chocolate chips, the butter, and vanilla until smooth. Transfer to the fridge to cool and thicken, at least 30 minutes.

Meanwhile, make the dough. In a food processor, combine the flour, cocoa powder, sugar, and butter.

Pulse until the mix clumps together to forms pea-size balls.

Add 1/2 cup cold water, 1 tablespoon at a time until the dough comes together and forms a ball. If the dough feels dry, add 1-2 tablespoons additional water.

If you don’t have a food processor, you can also do this by hand. Just rub the butter into the dry ingredients using your fingers until you have pea-sized balls.

Turn the dough out onto a floured surface. Roll out into a 1/8-inch thickness.

Cut the dough into rectangles, about 4 x 3 inches.

Place a tablespoon of the chilled chocolate fudge on one half of the rectangles, leaving a 1/4 inch border.

Brush the edges with the beaten egg. Lay the other half of the dough over the filling and seal the edges by crimping with the back of a fork.

Repeat until you've used all the dough, you will have leftover fudge.

Place the pop tarts on parchment-lined baking sheets. Cover the baking sheets and place in the fridge for 1 hour or the freezer for 20 minutes.

Preheat the oven to 400 degrees F. Bake the pop tarts for 15-20 minutes. The pop tarts will still leak a little. Cool completely before frosting.

Make the frosting: In a medium bowl, whisk together the powdered sugar, cocoa powder, vanilla, and 3 tablespoons water until smooth. Stir in the melted chocolate. If your frosting is too thick, thin with 1-2 tablespoons additional water.

Spoon a thin layer of the frosting on top of the cooled pop-tarts. Allow frosting to harden 10 minutes. Sprinkle with coarse sugar or salt.

Place on a baking sheet and allow the pop tarts to harden, uncovered, about 2 hours.