In a food processor, pulse the flour, sugar, and salt until blended. Add the butter and pulse until the mix is crumbly, about 2 - 3 pulses.
Add the egg and ½ cup of cold water and pulse until the mixture just comes together. You may need to add more water, but do so with only 1 teaspoon at a time.
Divide the dough into one flat disc. Wrap the disc in plastic wrap and place in the refrigerator until cold (about 30 minutes).
Once your dough is chilled, roll out on a lightly floured surface, until no more than ⅛ inch thick.
Using a large heart shaped cookie cutter, cut out individual heart shapes and place them on a parchment lined baking sheet. Alternatively, use a sharp knife to create equal size rectangles.
Preheat your oven to 375°F.
Place approximately 1 tablespoon of jam in the center of each heart. Don't overfill the tarts, you want them more flat than round.
In a small bowl, whisk the egg to create an egg wash. Brush the edges of each heart.
Cut out more heart shapes and top each filled heart with one. Seal the edges together with your fingers, then with a fork, crimp the edges together along the entire heart. Prick the fork in the center of each tart to allow steam to escape.
Repeat with remaining hearts.
Bake the pop tarts in your preheated oven for 20 minutes until the tops are lightly golden and you can see the jam bubbling out of the sides.
While the tarts are cooling, make the icing.
In a small bowl, mix together the powdered sugar, 2 tablespoons milk, vanilla. Add in more milk if needed. Keep the consistency thick enough so that the icing won't completely run off the pop tarts.
Add pink food coloring once you're happy with the consistency.
Using a teaspoon, spoon the icing onto the top of each pop tart and smooth it out with the back of the spoon. Top with rainbow sprinkles.