Make Pastry Dough: Combine flour, sugar, and salt in a medium-size bowl. Use your hands or a pastry cutter to incorporate butter and shortening into flour, with a few pea-sized pieces of butter remaining. In a small bowl, whisk together yolks and milk. Stir wet ingredients into dry ingredients with a spoon. Use your hands to fold the dough over on itself a few times. Shape into a 2-inch thick rectangle, wrap in plastic wrap, and refrigerate for 1 hour.

Make Filling: Combine filling ingredients in a small bowl with a fork until fully combined. Set aside.

Assemble Pop Tarts: Beat together egg and water; set aside. On a floured surface, roll dough into a 1/8-inch thick rectangle. Cut into eighteen 3- x 4-inch rectangles, re-rolling the scraps of dough as needed. Working with 1 small rectangle at a time, brush surface with egg wash, and spread 1 Tablespoon of Filling around the middle of the rectangle, leaving a 1/2-inch border. Brush another rectangle with egg wash and place the wet side on top of the brown sugar-covered surface. Press sides together and crimp the edges with a fork. Transfer to a parchment paper lined baking sheet. Repeat with remaining rectangles. Use a toothpick to prick a few holes in the center of each pop tart. Transfer baking sheet to refrigerator for 20 minutes.

Preheat oven to 350°F. Brush surface of each pop tart with egg wash and bake, rotating halfway through, until the edges become golden brown, about 25 minutes. Transfer pop tarts to a cooling rack and allow to cool completely before frosting.

Make Frosting: Combine powdered sugar and cinnamon. Add vanilla and 5 Tbsp. cream; mix until combined. Add more cream if needed to thin frosting. Frost pop tarts, and serve after frosting has dried slightly.