In a standing mixer, mix the flour, sugar, and salt. Add in the chilled butter and mix again until the mixture resembles coarse crumbs. Slowly add in the ice water, mixing with your hands. Work fast as to not melt the butter into the dough. Your goal is to keep everything as cold as possible! Stop adding water as soon as the dough comes together. If needed, fold the dough against the bowl a few times to ensure everything comes together.
Shape the dough into an oval disk and wrap in plastic wrap. Allow to chill in fridge for at least 1 hour, but up to overnight.
Make the poptarts
Remove the dough from the fridge. Place a large piece of saran wrap over a work surface, then sprinkle flour over it. Flour a rolling pin. Working quickly, roll the dough out into a large rectangle so the dough is about 1/4 of an inch thick. Using a ruler and a sharp knife, cut your dough into rectangles 1.5″ by 2.5″. take the remaining scraps of dough, form them into a ball, roll them out, and repeat until you’ve created 24 rectangles of dough.
Lift a corner of the saran wrap up off of your surface to help gently release the rectangles from their place. Transfer 12 rectangles to a large, parchment-lined baking sheet, leaving as little as 1/2 inch between each rectangle.
Whisk an egg in a small bowl, creating an eggwash. Brush each of your 12 rectangles with the wash. This will help the dough to stick together!
Spoon around 2 teaspoons of jam into the center of each rectangle. No need to spread it around. Place the remaining 12 rectangles dough over the base & filling of the rectangles, like a blanket. Press down slightly with your fingers to help seal the dough together. Then, using a fork, create indentations around the entire rectangle.
Brush the remaining eggwash over the poptarts, then use a fork to create small airholes on the top. Bake for 15 minutes, then allow to cool completely.
Make the icing
While the poptarts are baking, make your icing! Whisk together the powdered sugar, peanut butter, and milk. You’re looking for a thick but spreadable consistency (it shouldn’t look grainy at all). Once the poptarts are cooled completely, spread the icing over the poptarts, then sprinkle with chopped peanuts!