For the dough, add the flour, salt and butter to a food processor. Pulse until the mixture resembles wet bread crumbs. Add in the eggs and water and blend until a dough forms.
Pour the dough out and wrap with plastic wrap. Let sit in the fridge for 30 minutes to relax.
Roll out your dough on a floured surface until 1/4-inch thick. With a pizza cutter or knife, cut your pastry into 3x4-inch rectangles. You will need 2 for each pop tart. Brush the center of the bottom squares with marinara, add some cheese and then top with a few pepperoni.
Egg wash the pastry around the edge; this will help the 2 pieces stick together. Fold over the top and press the edges together; use a fork to crimp down the edges. Prick the top a few times with a fork to let steam out. Brush the tops with more egg wash.
Bake at 400°F for 22-25 minutes or until golden brown. Optional: in the last 10 minutes, top with more marinara, shredded cheese, and pepperoni. Finish baking until cheese is melted.