In a large bowl, combine the flour and salt. Add the chilled, diced butter and mix using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

In another bowl, mix the brown sugar, granulated sugar, and cinnamon. Set aside.

Gradually add milk to the flour mixture, stirring until a dough forms. Divide the dough into two halves, flatten them into disks, wrap them in plastic, and refrigerate for about 30 minutes.

Preheat your oven to 375°F (190°C). On a floured surface, roll out each disk to about 1/4 inch thick. Cut out rectangles, roughly 3 inches by 4 inches.

Place a tablespoon of the sugar filling onto half of the dough rectangles. Brush the edges with the beaten egg and top with the remaining dough rectangles, pressing down to seal.

Arrange the assembled poptarts on a baking sheet lined with parchment paper. Make a couple of small slits on the top of each poptart for steam to escape. Brush the tops with the remaining egg wash. Bake for 20-25 minutes or until golden brown.

Let them cool on a wire rack. If desired, add frosting on cooled poptarts for a sweet finish!