Combine flour, cocoa powder, granulated sugar and salt in the basin of a food processor and pulse briefly to combine.

Scatter cubed butter evenly over the surface of the flour mixture and pulse until the butter is cut into the mixture and mixture resembles coarse crumbs. Discernible pieces of butter may remain in the mixture, this is fine!

Add sour cream and pulse until dough is beginning to cling together. It may still be a bit crumbly but the flour and cocoa should be absorbed and if you press the dough between your thumb and forefinger it should stick together.

Remove from food processor and transfer to a piece of plastic wrap. Flatten into a disk and chill for at least 30-60 minutes before rolling out. In the meantime, prepare your filling.

Combine chocolate, butter, and cream in a heatproof bowl and heat in 25-second increments until melted and smooth.

Stir in vanilla extract and powdered sugar until smooth.

Set aside, filling should cool (and will thicken some) before being used to fill pastry.

Once pastry dough has chilled, preheat your oven to 400F (205C) and line a baking sheet with parchment paper.

Prepare egg wash by thoroughly whisking together egg and water.

Transfer pie crust to a clean, lightly floured surface and dust with additional flour as needed. Roll dough into a rectangle about ⅛-¼” thick and cut into 2.5”x4” (6x10cm) rectangles.

Place one rectangle on the prepared baking sheet and lightly brush the surface with egg wash.

Drop a rounded Tablespoon of chocolate filling into the center, then top with another rectangle of dough. Use a fork to crimp the sides to seal and brush the surface of the dough with egg wash. Use a toothpick or wooden skewer to poke 6-8 holes in the surface of the pop tart to allow it to vent.

Repeat the steps above until you have formed all of your chocolate pop tarts, then transfer to 400F (205C) oven and bake for 15-17 minutes. Allow to cool on baking sheet before decorating with icing.

Combine sugar, cocoa powder, milk, corn syrup, and vanilla extract and whisk until combined. If mixture is too thick, add additional milk as needed until smooth, and if icing becomes too thin, you can always thicken it with a bit more powdered sugar. Look for a consistency where, when you lift the spoon out of the icing, the ribbon of icing that drizzles back into the bowl holds its shape for several seconds before dissolving back in the icing.

Spoon icing over the center of the pop tarts and top with sprinkles, if using. Allow frosting to harden before serving and enjoying.