Step 1: Prepare the Dough

1. In a large mixing bowl, combine the gluten-free flour, almond flour, sugar, and salt.
2. Add the cold butter cubes and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
3. Gradually add cold water, 1 tablespoon at a time, mixing just until the dough comes together. Do not overmix.
4. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Step 2: Assemble the Pop Tarts

1. Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
2. Roll out the chilled dough between two sheets of parchment paper to about ¼-inch thickness.
3. Cut the dough into 12 rectangles (approximately 3×4 inches each).
4. Place a tablespoon of jam in the center of 6 rectangles, leaving a small border around the edges. If your jam is runny, mix it with the cornstarch slurry before filling.
5. Cover each jam-filled rectangle with another piece of dough. Use a fork to press and seal the edges. Prick the tops lightly with a fork to allow steam to escape.

Step 3: Bake the Pop Tarts

1. Transfer the Pop Tarts to the prepared baking sheet.
2. Bake for 12-15 minutes, or until the edges are lightly golden.
3. Let the pastries cool completely on a wire rack before glazing.

Step 4: Glaze and Decorate

1. In a small bowl, mix powdered sugar, milk, and vanilla extract until smooth.
2. Drizzle or spread the glaze over the cooled Pop Tarts. Add sprinkles if desired.
3. Allow the glaze to set before serving.