For the Filling

1. In a medium-sized saucepan over medium heat, stir cocoa powder, 1 cup of chocolate chips, and heavy cream together until smooth. Mixture will be slightly thick. Remove mixture from heat once fully combined.
2. Mix in sliced butter pieces and remaining 1/3 cup of chocolate chips. Mix until smooth- mixture will look thin. Allow the fudge to cool at room temperature for 5 minutes before transferring to the fridge to thicken for 30 minutes.

For the Dough 

1. In a large (preferably tall) bowl, whisk flour, cocoa powder, powdered sugar, and salt together.
2. Add thinly sliced butter pieces to the bowl, using your hands to squeeze the butter pieces into the flour mixture. Mix and continue squeezing for about 5 minutes until the mixture turns crumbly. There should be more crumbly pieces than flaky pieces. Be patient!
3. Add both egg yolks, mixing with a wooden spoon to combine.
4. Add water by the tablespoon until mixture becomes smooth and tacky. I used about 3 1/2 tablespoons of water. If it becomes sticky, add a teaspoon of flour until the dough returns to its drier texture.
5. Divide dough in half, create 2 disks of dough, and wrap each disk in plastic wrap. Place disks in the fridge for 30 minutes-1 hour.

For Assembly 

1. Remove one dough disk from the fridge. On a clean surface sprinkled with flour, roll the dough out using a rolling pin. Dough should be 1/8 inch thick… very thin. To ensure that the fudge filling really shines, do not go any thicker than 1/8 inch thick.
2. Cut dough into 3×4 inch rectangles using a sharp knife or pizza cutter. You should get about 4 rectangles from this first dough disk.
3. Repeat this process with your second dough disk.
4. Place the rectangles on the baking sheet.
5. Place no more than 1 1/2 tablespoons of fudge filling towards the center of half of the rectangles. Leave at least 1/2 inch of space around the outside of each rectangle to prevent leakage.
6. Combine your egg with your dash of water. Use a pastry brush, clean paint brush, or even a paper towel to brush your egg wash around the outside of the rectangles with fudge filling. I use these to brush the egg wash, but whatever you have will work!
7. Top each filled tart with one rectangle of dough, pressing around the outside of each pastry to squeeze the two layers together. Use a fork to crimp the edges, pressing down around the outside of each tart.
8. Preheat oven to 400 degrees Fahrenheit. Place tarts in the freezer for 10 minutes.

To Bake

1. Line baking sheets with parchment paper.
2. Remove tarts from the freezer, and place them at least two inches apart on the baking sheet.
3. Brush the tops of the pastries with the remaining egg wash.
4. Bake at 400 for 16-18 minutes.
5. Cool for at least 10 minutes before frosting.

For the Glaze

1. In a medium bowl, whisk powdered sugar and cocoa powder. Add milk, and stir to combine. Mixture should be smooth, slightly thick, and will harden when left untouched. Simply stir it to return it to its original consistency.
2. Top slightly cooled tarts with about 1 teaspoon of glaze. If using sprinkles, top immediately before the glaze hardens. Glaze will harden within about 10 minutes but will never fully harden.