In a large bowl, combine the flour, salt, and sugar. Add the sliced butter. Cover and let chill in the fridge (about 10 minutes) or freezer (about 5 minutes) until all of the ingredients are cold.
Using a pastry blender (or two forks), cut the cold butter into the flour mixture until it resembles coarse, pebbled sand. Add the ice cold water and stir to combine. Dough will be shaggy.
Dump the dough onto a lightly floured work surface and knead until the mixture comes together. Pat the dough to a thickness of 1/2-inch. Wrap in plastic wrap and let chill in the fridge for at least one hour (overnight is best).
MAKE THE FILLING:
In a small bowl, mix together brown sugar, flour and cinnamon. Add the molasses and stir together. Save the melted butter for assembly.
MAKE THE EGG WASH:
In a small bowl, whisk together the egg, water, and salt until smooth. Set aside until assembly.
ASSEMBLE THE POP TARTS:
Remove the dough from the fridge. Working on a lightly floured surface, roll the dough to a thickness of about 1/4-inch. Cut into rectangles 3 x 4-inches in size. Knead together scrapes and roll out dough again. You should end up with a total of 12 rectangles (6 tops and 6 bottoms).
Brush the melted butter on all 12 rectangles, making sure to butter up to the edges. Spoon a rounded tablespoon of filling into the center of 6 rectangles. Spread the filling, leaving a clean frame around the edges. Place the remaining rectangles (buttered side down) over the filled rectangles. Line up the edges of the top and bottom pieces, and then press the seams together. Use the tines of the fork to seal the edges.
Place the assembled pop tarts on a parchment-lined baking sheet. Freeze the pop tarts for at least 10 minutes until solid.
Preheat the oven to 350 degrees F. Prick the tops of the chilled pop tarts using a fork or toothpick. This will allow steam to release during baking. Lightly brush the tops of the pop tart with the prepared egg wash. Bake for 25 to 30 minutes until the pop tarts are golden brown in color. Let the pop tarts sit in the pan for 3 to 5 minutes before transferring to a wire rack to cool to room temperature.
PREPARE THE GLAZE:
In a medium bowl, whisk together the powdered sugar, milk, cinnamon and vanilla until smooth. Let the glaze sit for 5 minutes to slightly thicken. Drizzle the glaze over the cooled pop tarts. Serve immediately, or allow the glaze to set up and harden.
Notes
These pop tarts are best eaten the day they are baked. However, you can store they up for to three days at room temperature. Store in a container with a loosely fitted lid, or keep the lid slightly ajar. The glaze needs air circulation to prevent it from melting.