Preheat your oven to 350℉.
Make the cinnamon pie dough: In a large bowl, mix together the flour, salt, cinnamon, and granulated sugar. To that, add the cold cubed unsalted butter. Work the butter into the flour mixture with your hands until you get coarse crumbs. Then add in the water and mix until a dough forms. Pat into a rectangle shape and wrap in plastic wrap and set in the fridge for at least 1 hour.
Make the brown sugar cinnamon filling: In a bowl, simply mix together the dark brown sugar, cinnamon, flour, and salt. Set aside.
Once the dough is cooled, remove it from the fridge and cut in half. Roll out to ⅛" thin, as close to the shape of a rectangle as you can get. Cut into 9 rectangles. Move them onto a parchment lined baking sheet.
Add 1 heaping tablespoon of filling to the center of each dough rectangle, leaving ½" around the edge.
Repeat step 3 for the other half of dough, and place on top of the prepared halves. Using a fork, crimp down the edges. Brush with egg wash (1 beaten egg). Bake for 25-30 minutes.
Finish with cinnamon icing: combine the powdered sugar, milk, cinnamon, and salt in a bowl using a whisk or a fork until smooth. When the Pop Tarts come out of the oven, let them cool for at least 15 minutes, then drizzle with the icing.