Line a baking sheet with a piece of parchment paper.
In a large mixing bowl, whisk together the brown rice flour, tapioca or potato starch, sweet rice flour, xanthan gum, salt and 1 tablespoon sugar. Add the shortening and work it into the flour mixture with your hands until it is crumbly and the texture of coarse sand with some large pieces about the size of grapes. Add one egg and 2 tablespoons of rice milk. Mix with a spoon until you have a soft dough that is not sticky. You may need to work the dough with your hands to get it to come together. If too dry, add the final tablespoon of rice milk. Divide the dough into 2 equal portions and shape into rectangles. Wrap one portion of dough with plastic wrap and place in the refrigerator while rolling the first.
Place a piece of waxed or parchment paper on a work surface, sprinkle with some sweet rice flour, place one piece of the dough on the parchment, sprinkle with a little more sweet rice flour and top with another piece of paper. Roll into a rectangle that is a bit larger than 12 inches by 8 inches. Trim the dough with a knife into a 12 by 8 inch rectangle. Cut the 8 inch side in half and the 12 inch side into fourths to form eight 3 by 4 inch rectangles.
Beat the remaining egg and brush the bottom 4 rectangles with some of the egg. Place 2 heaping tablespoons of dessert filling on each bottom rectangle and shape it into a rectangle leaving ½ inch border on all sides. Slide your hand under the paper under the top rectangles and flip them so they are directly on top of the filled dough. Press the edges of the dough together and then press with the tines of a fork. Prick the top of each pop tart several times with the tines of a fork. Brush with more egg wash, sprinkle each pop tart with about a ¼ teaspoon of coarse sugar and place on the baking sheet lined with parchment paper. Place in the refrigerator and repeat with the final piece of dough. Refrigerate for at least ½ hour.
Preheat oven to 350 degrees while the pop tarts are in the refrigerator. Bake for 30 – 35 minutes or until golden brown. Let cool and serve.