The fudge filling needs to cool and thicken, so make it right after the dough. 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 cannabutter and vanilla extract. Pour into a heatproof bowl, cover, and set aside at room temperature to cool and thicken.
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. 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. 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 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.
Meanwhile, preheat oven to 375°F. 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. 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.