Preheat the oven to 350 degrees F (200 degrees C).
In a medium bowl, stir together the protein powder, almond flour, baking soda and salt.
Cut in the butter using a pastry blender or your fingers until the butter lumps are smaller than peas.
Stir in the water and sweetener to form a stiff dough.
On parchment paper, place 10 balls of dough a few inches apart. Shape with hands into rectangle shapes (size of a PopTart).
Beat the egg and brush it over the entire surface of the rectangles. Even the “insides” of the tart; the egg is to help glue the lid on.
Place a heaping tablespoon of desired filling into the center of each rectangle, keeping a bare 1/2-inch perimeter around it. Place a second rectangle of dough atop the first, using your fingertips to press firmly around the pocket of filling, sealing the dough well on all sides. Press the tines of a fork all around the edge of the rectangle. Repeat with remaining tarts.
Gently place the tarts on a lightly greased or parchment-lined baking sheet. Prick the top of each tart multiple times with a fork; you want to make sure steam can escape, or the tarts will become fluffy pillows instead of flat toaster pastries (my mistake number 1). Bake for 10-12 minutes in the preheated oven, until edges are lightly browned.
Cool in oven to crisp up.
While it bakes, prepare the glaze. In a medium sized bowl, place the 1/2 cup coconut oil (warm it until melted), extract and natural sweetener and mix well. Set aside to cool a bit. Spread over Pop Tarts. Serve.