Make the pastry dough: combine dry ingredients. Using a mixer with paddle attachment or your hands, work cold cubed butter into flour until about the size of peas (and all the flour has been moistened). Whisk egg into milk, add to flour/butter, and mix just until everything comes together. Shape into a flattened rectangle, wrap in plastic, and refrigerate for at least 30 minutes (or up to 5 days).
Preheat oven to 350˚F and line two baking sheets with parchment. Let dough sit at room temp for 15-20 minutes while you prepare your choice of fillings.
Roll out half of the dough to about ⅛ inch. Cut 2x3in rectangles (or other desired shape) and place on prepared baking sheets.
Roll out the rest of the dough and cut out matching shapes. You can reroll the scraps once if desired. Use a fork to poke holes into all of the cut out shapes.
Fill half with whatever fillings you like. Brush bottom side of remaining dough with egg wash and lay on top of filled pieces. Press edges gently with a fork to seal and brush tarts with egg wash. At this point you can refrigerate the pop tarts overnight or freeze up to a month.
Bake for 20-25 minutes, rotating pan halfway through, until golden brown.
Make glaze: whisk together confectioners sugar and milk. Allow pop tarts to cool completely before decorating with glaze and sprinkles (optional).
Recipe by Eat Some Wear Some at https://eatsomewearsome.com/pop-tarts-sweethearts/