In a bowl, combine all ingredients for the filling.

In a food processor, combine the flours, cinnamon, and butter. Pulse until the mixture forms pea-sized balls. Add cold water, 1 tablespoon at a time, until the dough forms a ball. If dry, add 1-2 tablespoons additional water.

Turn the dough out onto a floured surface and roll into 1/8-inch thickness. Cut into rectangles, about 4 x 3 inches. Place a rounded tablespoon of filling on one half, leaving a 1/4 inch border. Brush edges with beaten egg. Fold the other half over the filling and seal the edges by crimping with a fork. Repeat with all dough.

Place pop tarts on parchment-lined baking sheets. Poke a few holes on top with a fork. Cover and chill in the fridge for 1 hour or freeze for 20 minutes.

Preheat the oven to 400 degrees F. Bake pop tarts for 15-20 minutes, until golden. Cool completely before frosting.

In a medium bowl, whisk together powdered sugar, cinnamon, vanilla, and 3 tablespoons water until smooth. If too thick, thin with 1-2 tablespoons additional water.

Spoon a thin layer of frosting on top of cooled pop tarts. Allow to harden, uncovered for about 2 hours. Sprinkle with salt or sugar if desired. Store in an airtight container for up to 3 days.