To make the pop tart pastries:

• Make the pastry dough. Add the flour, powdered sugar and kosher salt to the bowl of a large food processor and pulse until ingredients are combined. Next, add the butter to the bowl and turn the processor on low speed, mixing until the mixture resembles fine bread crumbs.
• In a small bowl, combine the egg yolk and milk. Pour the egg mixture over the flour/butter mixture. Mix on low speed until the ingredients are just combined and the dough starts to come together — the dough will be crumbly, but should hold together when pressed.
• Turn the dough out onto a lightly floured work surface. Press together and knead by hand until a smooth dough forms, being careful not to overwork the dough (You might think the dough is too dry at first, but be patient! It will come together!). Divide the dough in half. Shape each half into a rectangle, 1-inch thick.  Wrap each rectangle tightly in plastic wrap and place in the fridge for at least 30 minutes, up to overnight (for longer chill times let the dough sit at room temperature for 10 minutes before rolling out).
• Make the jam filling. Stir the cornstarch and water together in a small bowl. Add the cornstarch slurry and the jam to a small saucepan set over medium-high heat, stirring together as the jam warms and the mixture becomes a liquid. Bring to a full boil, then reduce heat to medium-low and continue to simmer, stirring constantly, for 2 minutes. Pour into a heatproof bowl and set aside to cool.
• Line a large baking sheet with parchment paper; set aside. Make the egg wash by combining 1 egg with 1 tablespoon water; set aside.
• Before rolling out your dough, I recommend scrolling up and reading my tips on working with short crust pastry before proceeding.
• Roll out the pastry dough. Remove one portion of dough from the fridge. On a heavily floured work surface, roll the dough into a 10×13″ rectangle, about 1/8″ thick. Trim the edges to create a neat 9×12″ rectangle. Next, cut the rectangle into nine, 3×4″ rectangles. Place each rectangle on the prepared baking sheet and place in the fridge to chill.
• Next, remove the second portion of dough from the fridge. Repeat the process of rolling out and cutting into rectangles, just as you did with the first portion of dough.
• Assemble the pop tarts. Brush  the chilled rectangles on the baking sheet with egg wash, making sure to brush the entire surface, all the way to the edges. Divide the prepared jam between the rectangles, spooning about 1 tablespoon onto the center of each rectangle. Next, brush the remaining rectangles with egg wash (again brushing all the way to the edges), and place them, wet side down, onto the rectangles with jam. Use your fingers to seal the rectangles together, pressing around the edges of the filling and out to the edges of the rectangle. Next, use a fork to crimp the edges. Place the tarts in the fridge for 30 minutes. Meanwhile preheat the oven to 350°F.
• Bake! Brush the chilled tarts with egg wash and prick the tops multiple times with a fork (this allows steam to escape when they’re in the oven).Bake for 25 to 30 minutes, until golden. Allow to cool for 5 to 10 minutes on the pan before removing to a wire rack to cool completely.

To make the icing:

• Add the powdered sugar, vanilla extract, and 3 tablespoons milk or heavy cream to a medium bowl and whisk to combine. If necessary, add more milk, 1 teaspoon at a time, until you’ve reached a thick, drizzling consistency. Stir in food coloring, if using. Spoon the glaze over the cooled pop tarts. Top with sprinkles, if desired.