• Freeze the butter until frozen solid, about 1 hour. You can also do this the night before.
• Then rub the butter against the largest grating surface on a box grater or cheese grater, catching the butter in a bowl or plate. Place the butter back in the freezer while you measure the rest of the ingredients.
• In a stand mixing bowl, combine the dry ingredients for the pie dough.
• Then add the frozen shredded butter. Mix with the paddle attachment until the butter is coated in flour.
• Measure the water and vinegar into a bowl and then add ice. Stir to melt some of the ice so that the water is super cold.
• With the mixer running on low, drizzle in 1 tablespoon of the ice water at a time. You will need about 12-15 tbsp. Stop adding water when the dough comes together easily and forms large clumps. The dough should feel moist and a little sticky, but not feel overly wet. Do not add more water than you need to!
• Portion the dough in half, about 330 grams each. Wrap each half loosely in plastic wrap, then use a rolling pin to roll each into a flat square disc. Chill the pie dough in the fridge for at least 1 hour or up to 5 days before using.

Make the Jam

• Combine the rhubarb, strawberries, sugar, and lemon juice in a pot. Cook over medium heat for 22-25 minutes, stirring and mashing occasionally, until the jam has reduced and thickened. It will be a dark red color when it's ready. You should have about ¾ cup of jam.
• Transfer the jam to a bowl to cool completely before using. Reserve 1 tablespoon of the jam for the icing.

Assembly

• Line a baking sheet with parchment paper.
• Take one disc of dough from the fridge and place it on a lightly floured surface. Lightly dust the top of the dough and your rolling pin with flour.
• Start from the center and work your way out in all directions, turning the dough with your hands as you go. Between passes of the rolling pin, rotate the pie crust and even flip it, to make sure it’s not sticking to your work surface. Sprinkle on a little more flour if it’s sticking.
• Roll it into a rectangle about ⅛" thick and 9"×12" in size. Trim the sides as needed. Always be gentle with your pastry dough so you don't tear it. Cut the dough into fourths and then in half. You will end up with 8 rectangles, each measuring about 3"×4". Use a ruler to help make this process easier and more accurate!
• Place each of the 8 rectangles onto the prepared baking sheet. The pop-tarts will not spread in the oven much, so you may place them relatively near one another. Place the baking sheet in the fridge. Roll the second disc out into a rectangle and cut it into 8 even rectangles as you did with the first half of the dough. These 8 rectangles will be the tops of your homemade pop-tarts.
• Remove the baking sheet of rectangles from the refrigerator. Brush egg wash over the entire surface of each rectangle. These will be the bottoms of your pop-tarts and the egg wash will help glue the top on. Place a heaping tablespoon of the strawberry rhubarb filling into the center of each rectangle, and spread it around, leaving some space on the edges. Then place the other half of the rectangles on top of the filling-topped rectangles. Use your fingertips to press firmly around the pocket of the filling, sealing the dough well on all sides.
• Poke holes in the tops of each filled pastry to allow the steam to escape. This helps get your pop-tart pastry nice and flaky. Seal the edges by crimping or indenting with a fork, to prevent the sides from opening as the pop-tarts bake. I like to stick a fork straight into the dough but you can also keep the fork flat for a traditional crimped edge!
• Freeze the filled pop-tarts uncovered for at least 20 minutes and up to 1 hour. This chilling let the pop-tarts rest before baking.
• Preheat oven to 375°F (190°C). Once the unbaked pop-tarts have chilled for 20 minutes, remove them from the freezer and brush the tops with the remaining egg wash. This egg wash will give your pastry that beautiful golden sheen. Bake for about 22-26 minutes or until the tops are golden brown.
• Let the baked pop-tarts cool on the pan for about 5 minutes, then transfer to a wire rack to cool completely before glazing.
• Push the reserved tablespoon of strawberry jam through a fine mesh sieve (to get rid of the seeds). If you don't mind the seeds, feel free to skip this step. Then whisk together the glaze ingredients until it is a smooth and spreadable consistency.
• Spoon the glaze over the top of the pop tarts and spread it evenly around the top. Immediately garnish with sprinkles if using. Don't wait to sprinkle them or the glaze will set and the sprinkles will roll right off!
• The glaze will set in about 1-2 hours depending on how humid your kitchen is. You may be tempted to pop it in the fridge or freezer to set quickly but this will wrinkle the glaze!