In a food processor, combine the flour (make sure to spoon and level), salt and sugar. Pulse to combine.
Cube the butter and add to the food processor.
Process until well combined with the flour, about 20 seconds.
With the mixer running, add in the cold water, starting with 3 tablespoons and adding more until the dough comes together and looks like a pie dough. You don't want it too wet, so add the water slowly.
Once the dough is ready, transfer to plastic wrap and wrap tightly. Chill for 30 minutes.
Preheat the oven to 375 Fahrenheit.
To a floured piece of parchment paper, add half of the dough and roll into a 9x10 rectangle using a floured rolling pin. I like to do it slightly larger, so I can trim the edges and make a perfect rectangle. You can also choose to roll the dough a bit thinner and make 8-9 poptarts instead.
Cut into 6, 3x5 rectangles.
Add to a baking sheet lined with parchment paper.
Take the other half of dough and repeat, but do not add to the parchment paper.
Take about 2 tablespoons of jam and place in the center of the rectangles that are on the parchment paper. Spread slightly, but leave enough space to seal the edges.
Take a second rectangle without the jam and place on top of the jam filled one.
Lightly wet a fork and dip in flour. Seal the edges with the fork around each poptart. Repeat with all poptarts.
Whisk up the egg and brush on top of each poptart.
Bake for 22-28 minutes or until lightly golden brown on top.
Meanwhile, whisk together all ingredients for the icing, add 1-2 more teaspoons of milk if needed to thin.
Once the poptarts are done, let cool completely, then top with the icing and sprinkles and enjoy!