Make the crust: In a food processor, pulse flour, cocoa, sugar, and salt. Add cold butter cubes and pulse until it looks like coarse crumbs. Add sour cream and pulse again until the dough just starts to stick together. Flatten into a disk, wrap in plastic, and chill for at least 30 minutes.
Prepare the filling: In a microwave-safe bowl, heat chocolate chips, butter, and cream in 25-second bursts, stirring until smooth. Stir in vanilla and powdered sugar. Set aside to cool and thicken.
Preheat & roll: Set oven to 400°F. On a lightly floured surface, roll the chilled dough to about 1/8–1/4 inch thick. Cut into rectangles (about 2.5x4 inches).
Assemble: Place one rectangle on a parchment-lined baking sheet. Brush lightly with egg wash. Add a rounded teaspoon of filling in the center. Top with another rectangle. Press edges with a fork to seal. Brush the top with more egg wash. Poke 6–8 small holes in the top with a toothpick.
Bake: Bake for 15–17 minutes. Let cool completely on the baking sheet.
Glaze & decorate: Whisk together powdered sugar, cocoa, milk, corn syrup, and vanilla until smooth. Spoon over cooled pop tarts. Add sprinkles right away. Let the glaze set before packing.