Place the flour, cocoa powder, sugar, and salt into a large 8-cup capacity or higher food processor or, if you don’t have a food processor, in a large bowl. Pulse or whisk until combined. Add butter and pulse or use a pastry cutter to manually cut in until coarse pea-sized crumbs form. See photo above for a visual. In 3 additions, pour in the milk and pulse or stir after each addition. After all the milk is added, pulse or stir more until the dough clumps up and comes together. If it’s too dry, add 1 Tablespoon (15ml) of cold milk. Try not to overwork the dough. Pour onto a work surface lightly dusted with cocoa powder or all-purpose flour. Use your hands to form dough into a ball. Divide the dough in half and flatten each into 1-inch thick discs. Wrap each disc tightly in plastic wrap and refrigerate for 2 hours and up to 5 days.

The fudge filling needs to cool and thicken, so that is why I recommend making it soon after you put the pastry dough in the refrigerator. Place chocolate, milk, and sugar in a medium saucepan over medium heat. Stir frequently as chocolate melts and mixture combines. Once melted, remove from heat and stir in the butter and vanilla extract. Pour into a heatproof bowl, cover, and set aside at room temperature to cool and thicken. You want the consistency slightly thinner than peanut butter. Note: You can refrigerate the filling to thicken it, but it may become too thick and cold. If it’s too thick, reheat in the microwave for a few seconds.

Line 1 large baking sheet with parchment paper or a silicone baking mat. Make sure there is enough room in your refrigerator for the baking sheet because the shaped pastry pop tarts must chill for 15 minutes before baking.

Remove 1 of the dough discs from the refrigerator and let it sit at room temperature for 10 minutes to slightly warm up because it will be quite solid. (Keep the other in the refrigerator until step 5.) Lightly dust a work surface, rolling pin, and your hands with cocoa powder or all-purpose flour. Using a rolling pin, roll the dough into a 9×12-inch rectangle. Use gentle force with your rolling pin and use your hands to help keep the edges straight. You can watch me roll out the dough in the video below if you need a visual (2:23 mark). Visible specks of butter and fat in the dough are perfectly normal and expected. Trim the edges of the rectangle to straighten if needed. Cut rectangle into 3 equal strips and cut those into thirds for a total of 9 3×4-inch rectangles. Use a ruler to help make this process easier and more accurate. Arrange the dough rectangles 2 inches apart on the prepared baking sheet and chill in the refrigerator as you roll out 2nd half of dough in the next step.

Roll out the 2nd half of dough and cut into 9 3×4-inch rectangles like you did with the 1st half of dough in step 4.

Remove 1st set of dough rectangles from the refrigerator. Using a pastry brush, brush edges of each of these 9 rectangles with egg wash. Spoon a heaping Tablespoon of filling into center of each. Lay the other set of pie dough rectangles on top of each and press your finger around the edges to slightly seal. Poke holes in the top pie crust with a fork or toothpick and then crimp with a fork to ensure no filling spills out (if it does, it’s not a big deal). Refrigerate the shaped pastries uncovered for at least 15 minutes and up to 3 hours. This firms up the pastry and helps set its shape. (After chilling, feel free to carefully transfer the pastries to a new lined baking sheet if you don’t want to bake them on the cold baking sheet.)

Meanwhile, preheat oven to 375°F (191°C). And look ahead to step 9—you can make the icing as the pastries bake in step 8.

Brush pastry tarts with egg wash and bake for 25-26 minutes or until edges appear set. Remove from the oven and allow pastries to cool for 5 minutes before transferring to a wire rack to cool completely. Pastries taste best after completely cooling—they are very soft and rich when warm.

In a medium saucepan over low heat or in a medium heatproof bowl using the microwave, warm butter and water together until butter has melted. Whisk mixture with the remaining icing ingredients. Cool icing for 15 minutes before spooning onto pastries. You can add the icing when the pastries are still warm, but again, they taste best after completely cooling. Immediately top with sprinkles, if using. Note: The longer the icing cools, the thicker it gets. If you let it cool too long before spooning on the pastries, the surface will stiffen up—whisking will bring it back together though. You can also reheat on the stove or in the microwave to thin out if needed.

Cover and store pastries at room temperature for up to 1 day or in the refrigerator for up to 1 week.