Start with the fudge sauce. In a small saucepan over medium-low heat, stir together chocolate chips, sugar, cocoa powder, Kahlua (if using), and vanilla until melted, about 5 minutes. Remove from heat and transfer to a bowl or Pyrex measuring glass. Refrigerate until chilled, about 10 minutes. You may have fudge sauce leftover from filling later on.
In a food processor, combine flour, cinnamon, and salt. Pulse cubes of butter into the flour mixture until fine crumbs, no larger than the size of a pea, are formed. Slowly stream in cold milk until a cohesive dough ball forms. It may be somewhat shaggy, and should not be sticky at all. If you don't have a food processor, you can perform these steps by hand. Cut the butter with your fingers, two forks, or a pastry cutter.
Generously flour a clean, flat surface. Roll dough into an approximately 9 x 13 inch rectangle at 1/8 inch thickness. Using a sharp knife or pizza cutter (what I recommend), slice 18 rectangles about 3 x 4 inches. Lightly beat the egg and brush around the border of each rectangle.
Remove fudge sauce from the fridge. Dollop a rounded tablespoon of sauce into the center of 9 of the rectangles. Set an unfilled rectangle overtop each filled rectangle., Use a fork to crump the perimeter of each together.
Line two baking sheets with parchment paper. Set poptarts on the prepared sheets and transfer to the fridge to chill at least an hour, or the freezer for 20 minutes.
When the poptarts are ready, preheat oven to 400 degrees. Bake 15-18 minutes until poptarts are golden. Let cool on the baking sheets for about 20 minutes.
After the poptarts have cooled, whisk the sugar and cinnamon together in a shallow dish. To coat, brush the top of a optart with butter, then dip into the cinnamon sugar and press gently to thoroughly cover the surface. Brush the bottom with butter and dip. Repeat with remaining poptarts.
If you want, toast each poptart at 350 degrees for about 5 minutes before serving.