Add the flour, 1 tablespoon of powdered sugar, and salt to a food processor.
Cut the stick of cold butter into chunks, and then add it to the food processor.
Pulse until the mixture forms a soft crumb.
Gradually add in 8 tablespoons of cold milk, and mix until a soft dough forms.
Once your dough is ready, knead it in your hands to form a ball, and then wrap it in plastic wrap. Put the dough in the fridge to chill for 1 hour.
While the dough chills, make the filling. Add ½ can of pumpkin to a mixing bowl. Add in ⅔ cup of brown sugar, 2 tablespoons of powdered sugar, cinnamon, and nutmeg. Mix to combine.
Preheat the oven to 350 F.
Prep a flat surface such as a countertop with plenty of flour to prevent the dough from sticking while rolling it out.
Flour a rolling pin, and then roll out the chilled dough, about ¼-inch thick.
Use a pastry cutter, a pizza cutter, or a knife to cut 8 even rectangles out of the dough. You may need to re-roll the dough. Transfer 4 of the rectangles to a baking sheet lined with parchment paper or sprayed with cooking spray.
Scoop the pumpkin filling into the middle of each rectangle. Be sure not to get the filling too close to the edge of the dough.
Once each rectangle has filling, top each with the remaining cut dough pieces. Use your thumbs to pinch the outer edges of each PopTart, and use the prongs at the end of a fork to finish pinching.
In a small bowl, whisk together 1 egg yolk and 1 tablespoon of water.
Brush the top of each piece of dough with the egg wash.
Bake for 20 minutes until lightly golden. Then, remove the PopTarts from the oven to cool.
Make the glaze by adding the 3 cups of powdered sugar and 1 tablespoon of brown sugar to a bowl, and whisk. Add in 3 tablespoons of milk, and continue whisking until a thick glaze forms. Whisk well to avoid any clumps in the icing.
Once the PopTarts are fully cooled, spread the glaze on top of each.
Decorate with sprinkles, if desired, and allow the icing to finish setting.