In a medium bowl or the bowl of a stand mixer, mix together the flour and salt. Grate the cold butter into the flour mixture, and cut in using a pastry blender or the paddle attachment, until the pieces of butter are no larger than pea sized. Add the buttermilk, and mix until the dough just starts to come together.
Turn the dough out onto a floured surface and gently work into a ball, handling as little as possible. Divide the dough in half and shape into discs. Wrap in plastic wrap and refrigerate at least twenty minutes, or until ready to use.
Preheat the oven to 350. Line two baking sheets with parchment or silicone liners and set aside. Whisk the egg and milk together in a small bowl.
Roll the discs out to ⅛ inch thick on a floured surface. Cut the dough into rectangles using a pizza cutter (mine were about 3 inches by 4 inches). Brush half of the rectangles with the egg mixture, then spoon 1 teaspoon of preserves onto the center of the brushed rectangles.
Using a chopstick, poke holes in the other half of the rectangles to allow steam to escape. Place the pieces with holes on top of the bottom pieces, and crimp with a fork to close. Brush the tops with the egg mixture.
Bake 25 minutes, or until golden brown on top. Remove from oven and let cool completely. While the pop tarts are cooling, whisk together all ingredients for the glaze except the sprinkles. Spoon the glaze over the cooled pop tarts and top with sprinkles if desired.