Make the Gluten-Free Pastry Dough:
* Combine Dry Ingredients: In a large bowl, whisk together the gluten-free flour blend, granulated sugar, and salt. If your flour blend does not contain xanthan gum, whisk in 1 teaspoon now.
* Cut in Fats: Add the very cold cubed butter and cold shortening to the flour mixture. Using a pastry blender, two knives, or your fingertips, quickly cut the fats into the flour until the mixture resembles coarse meal with some pea-sized pieces of fat remaining. Work quickly to prevent the fats from melting. Alternatively, pulse briefly in a food processor (be careful not to over-process).
* Add Ice Water: Sprinkle about ½ cup of the ice water over the flour/fat mixture. Gently mix with a fork or your hands just until the dough starts to come together. If it seems too dry and crumbly, add more ice water, 1 tablespoon at a time, until the dough just holds together when squeezed. Avoid over-mixing, which can make the crust tough. Gluten-free doughs can be deceivingly dry at first – give it a moment to hydrate.
* Divide and Chill (Crucial): Divide the dough into two equal halves. Gently shape each half into a flat disk (about 1-inch thick). Wrap each disk tightly in plastic wrap. Refrigerate for at least 1 hour, or up to 2 days. Chilling is essential for hydrating the gluten-free flours and solidifying the fats, making the dough easier to handle and promoting flakiness.

2. Prepare Filling (if making Brown Sugar Cinnamon):
* While the dough chills, if using the brown sugar cinnamon filling, combine the packed light brown sugar, 1 tablespoon of gluten-free flour, and ground cinnamon in a small bowl. Stir in the melted butter until a thick paste forms. Set aside.

3. Assemble the Pop Tarts:
* Preheat Oven & Prepare Pans: Preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper.
* Roll Out Dough: Remove one disk of chilled dough from the refrigerator. Place it on a lightly floured piece of parchment paper (using GF flour). Lightly flour the top of the dough and place another piece of parchment paper on top. Roll the dough out between the parchment sheets into a rectangle roughly 1/8-inch thick (about 10×14 inches, dimensions approximate). Gluten-free dough can be prone to cracking, especially at the edges – roll gently and evenly. Tip: If the dough cracks badly, it might be too cold; let it sit at room temp for 5-10 minutes. If it’s too soft/sticky, chill briefly.
* Cut Rectangles: Remove the top parchment paper. Using a sharp knife or a pastry wheel, trim the edges to create a neat rectangle. Cut the dough into smaller rectangles, approximately 3×4 inches each (you should get about 8-10 rectangles from both disks combined). Carefully transfer half of the rectangles (these will be the bottoms) to the prepared baking sheets using a thin spatula, leaving space between them. Gather scraps, re-roll once if possible (GF dough doesn’t tolerate much re-rolling), and cut more rectangles. Repeat the rolling and cutting process with the second disk of dough (these will be the tops).
* Add Filling: Spoon about 1-1.5 tablespoons of your chosen filling onto the center of each bottom rectangle, leaving a ½-inch border clear all around. Do not overfill, as this is the primary cause of leaking during baking. Spread the filling slightly, keeping it away from the edges.
* Prepare Egg Wash: In a small bowl, beat the egg with 1 tablespoon of milk or water.
* Seal the Pop Tarts: Lightly brush the clear border of the bottom rectangles with the egg wash (this acts as glue). Carefully place a top rectangle over each filled bottom rectangle. Gently press down around the edges with your fingertips to seal.
* Crimp and Vent: Using the tines of a fork dipped lightly in GF flour, press firmly around all four edges of each pop tart to create a decorative crimp and ensure a good seal. Using the fork or a sharp knife, prick the top of each pop tart 2-3 times to allow steam to escape during baking (venting).
* Final Chill (Optional but Recommended): For best results and to prevent spreading, place the assembled pop tarts (on their baking sheets) in the freezer for 15 minutes or the refrigerator for 30 minutes before baking.

4. Bake the Pop Tarts:
* Brush the tops of the chilled pop tarts evenly with the remaining egg wash (this gives them a golden colour and slight sheen).
* Bake in the preheated oven at 375°F (190°C) for 20-25 minutes, or until the edges are golden brown and the tops are lightly browned. Rotate the baking sheets halfway through baking for even cooking.
* Remove from oven and let the pop tarts cool on the baking sheets for 5-10 minutes before carefully transferring them to a wire rack to cool completely. They will firm up as they cool.

5. Make the Glaze and Decorate (Optional):
* Once the pop tarts are completely cool, prepare the glaze. In a medium bowl, whisk together the sifted powdered sugar, vanilla extract (if using), and 2 tablespoons of milk/water/lemon juice. Add more liquid, 1 teaspoon at a time, whisking until you reach your desired drizzling or spreading consistency.
* Spoon or spread the glaze over the cooled pop tarts. Immediately add gluten-free sprinkles, if desired, before the glaze sets.
* Let the glaze set completely before storing or serving.