Homemade Frosted Brown Sugar Cinnamon Pop Tarts

1. To make the filling. Combine all ingredients in a bowl. 
2. To make the dough. In a food processor, combine the flour, whole wheat flour, cinnamon, and butter. Pulse until the mix clumps together to form pea-size balls. Add 1/2 cup cold water, 1 tablespoon at a time, until the dough comes together and forms a ball. If the dough feels dry, add 1-2 tablespoons additional water.
3. Turn the dough out onto a floured surface. Roll out into a 1/8-inch thickness. Cut the dough into rectangles, about 4 x 3 inches. Place a rounded tablespoon of the brown sugar filling on one half of the rectangles, leaving a 1/4 inch border. Brush the edges with the beaten egg. Lay the other half of the dough over the filling and seal the edges by crimping with the back of a fork. Repeat until you've used all the dough.
4. Place the pop tarts on parchment-lined baking sheets. Poke a few holes on top of the tarts with a fork. Cover the baking sheets and place in the fridge for 1 hour or the freezer for 20 minutes.
5. Preheat the oven to 400 degrees F. Bake the pop tarts for 15-20 minutes, until golden. Cool completely before frosting.
6. Meanwhile, make the frosting. In a medium bowl, whisk together the powdered sugar, cinnamon, vanilla, and 3 tablespoons water until smooth. If your frosting is too thick, thin with 1-2 tablespoons additional water.
7. Spoon a thin layer of the frosting on top of the cooled pop-tarts. Place on a baking sheet and allow the pop tarts to harden, uncovered for about 2 hours. Sprinkle with salt or sugar, if desired. Store in an airtight container for up to 3 days.