Make the Dough:

1. Add the flour, powdered sugar, and salt to the bowl of a food processor. Pulse until combined. Add in butter and pulse 7 times. With the motor running, pour in the milk and let process until the dough forms together into a cohesive ball.
2. Scrape the dough from the bowl and place it in a piece of plastic wrap and form into a rectangle, and wrap tightly. Using a rolling pin roll the dough out until it stretches to the corner of the plastic wrap to ensure a tight seal.
3. Refrigerate for at least 2 hours, but preferably overnight.

Make the Filling:

1. Combine all the ingredients together in a bowl.

Prepare the Pop Tarts:

1. Remove the chilled dough from the refrigerator and let rest at room temperature for 5-10 minutes, or until the dough is slightly softened, so it is easier to roll. Divide the dough in half.
2. On a well-floured work surface, roll out one piece of dough to a rough 9 x 13 rectangle, about 1/8” thick. Using a pastry wheel or a knife, cut out 3 x 4-inch rectangles. Repeat with remaining dough. Then gather up the scraps and roll out the dough in the same manner, cutting out as many rectangles as possible. This dough should yield 16-18 rectangles.
3. Line a baking sheet with parchment paper and place 9 rectangles down on the baking tray.
4. Whisk together the egg and the milk to make the egg wash. Brush the egg wash on top of the edges of the pastry.
5. Place 1 heaping tablespoon of brown sugar filling in the middle of all of the rectangles. Gently spread out the sugar until it is evenly distributed, leaving a ½” border around the edges of the pastry. Place the remaining pastry rectangles on top, taking care to seal in the edges. Take a fork and crimp the edges. Finally, using a pastry bench scraper, press down to cut off the edges of the pastry. This makes the final product look neat, but it also seals the pop tarts well.
6. Chill the tray of pop tarts in the refrigerator for at least 30 minutes. Once chilled, using a knife, gently pop holes through the top of the dough to let the steam escape during baking.

Bake and Frost the Pop Tarts:

1. Preheat the oven to 375 degrees. Bake the pop tarts in the middle rack, until the bottoms are slightly golden, about 25 minutes. Transfer to a rack and let cool.
2. Add the powdered sugar, cinnamon, milk, and vanilla to a bowl and whisk together. The glaze should be thick enough that it will keep its shape for a few seconds when drizzled off of a spoon.
3. Place 1 tablespoon of glaze in the middle of the pop tarts and gently smooth it down.