Make the chocolate dough: 

1. Add the flour, cocoa powder, powdered sugar and salt to the bowl of a food processor. Pulse until combined. Add in butter and pulse until the butter is broken down into rice-sized pieces, about 7-10 times. With the motor running add in the eggs and let the dough process until it forms together into a cohesive ball around the blade.
2. Remove the dough from the food processor and form it into a rough rectangle. Wrap the dough tightly in plastic wrap and let it chill in the refrigerator for at least two hours, and up to 3 days.

Make the fudge filling:

1. Add milk, chocolate chips, salt, and brown sugar to a medium heatproof bowl. Set the bowl over a small simmering pot of water and stir the mixture until it’s melted.
2. Remove the bowl from the heat, and whisk in the butter.
3. Transfer the fudge filling to a shallow container, and let cool for 30 minutes at room temperature, before transferring to the refrigerator to chill for 1-½ hours, or until very cold and thick.
4. Transfer the fudge filling to a piping bag or a ziploc bag with the corner snipped off.

Prepare the Pop Tarts:

1. Remove the chilled dough from the refrigerator and let rest at room temperature for 5-10 minutes, or until the dough is slightly softened, so it is easier to roll. 
2. Cut the piece in half. On a lightly floured work surface, roll out one piece of dough to a rough 10 x 13 rectangle, about 1/8” thick. Using a pastry wheel or a knife, cut out 3 x 4-inch rectangles. Repeat with remaining dough. Then gather up the scraps and roll out the dough in the same manner, cutting out as many rectangles as possible. This dough should yield about 18-20 rectangles.
3. Line a baking sheet with parchment paper and place half the rectangles down on the baking tray.
4. Whisk together the egg and the milk to make the egg wash. Very lightly brush the egg wash on the pastry squares.
5. Pipe about 1 heaping tablespoons worth of fudge filling in the middle of the pastry squares, taking care to leave a ½” border around the edges of the pastry. Place the remaining pastry rectangles on top and seal the edges. Using a pastry bench scraper, cut off the very ends of the sealed edges. This makes the final product look neat, but it also seals the pop tarts well.
6. Chill the tray of pop tarts in the refrigerator for at least 30 minutes, or in the freezer for 15. Once chilled, using a knife, gently pop holes through the top of the dough to let the steam escape during baking.

Bake and Frost the Pop Tarts:

1. Preheat the oven to 375 degrees. Bake the pop tarts on the lower rack for 20 minutes. The pop tarts should be fragrant and the pastry will be fully set, and even some of the pastry slightly starting to get cracks as the fudge filling puffs up. As soon as possible, transfer to a wire rack and let cool until no longer hot to the touch, about 1 hour.
2. Add the powdered sugar, cocoa, milk, and espresso powder to a bowl and whisk together. The glaze should be thick enough that it will keep its shape for a second when drizzled off of a spoon.
3. Place 1 tablespoon of glaze in the middle of the pop tarts and gently smooth it down. Decorate the pop tarts with sprinkles before the glaze sets.