Combine flour, salt, and sugar in a bowl and mix together. Add the cold butter and use your hands (or a food processor) to incorporate. Once the butter resembles pea size pieces, it's ready.
Start by adding 4 tbsp of water first and mix together. If the mixture isn't clumping together, add another tbsp of water.
Flour your counter and drop the mixture onto it. Divide the dough into two pieces and shape each piece into a 5-inch disk. Wrap in plastic wrap and chill for 1 hour.
Line two baking sheets with parchment paper. Remove 1 disk from the fridge and roll it out onto a floured surface to a 13 x 11- inch rectangle. Trim down with a pizza cutter to get a clean 12 x 10- inch rectangle. Cut the dough into 5 x 3-inch rectangles. There should be 8 in total. Place them on the prepared sheet and place in fridge.
Grab the second disk and repeat the process. Place the 8 pieces on the prepared sheet and place second tray in the fridge. While the dough chills, begin the filling.
Combine brown sugar, cinnamon, and flour in a bowl and set aside. In another bowl combine egg and milk to make egg wash.
Take 1 of the baking sheets from fridge and brush each of the rectangles with the egg wash. Place 1 tbsp of filling in the center of each rectangle and spread evenly. Leave about 1/4 inch of space around all the edges. Brush the rectangles on the 2nd baking sheet and then place (egg wash side down) on top of the rectangles with filling. Press down on the edges to seal the pop tarts shut.
Seal all edges with a fork to get a nice crimping look and a tiny hole into the top of each pop tart using a toothpick. Place trays in the fridge for at least 20 minutes.
Preheat oven to 350°F. Brush the tops of the pop tarts with egg wash. Bake for 22 to 25 minutes or until golden brown. Rotate the pan halfway through. Let cool completely.
Mix powdered sugar, vanilla, cinnamon, and milk in a bowl until it's thick enough to spread. If too thick, add 1 tsp or milk. Spread over cold pop tarts. Enjoy pop tarts right away or wait until the icing hardens!
Serving Size 1 pop tart