Ever since I saw the Crepes of Wrath's S'mores Pop-Tart slab pie, I just knew I had to make my own version of a Frosted Strawberry Pop-Tart! Can you think of anything more fun and delightful than a huge slab pie filled with jammy fresh strawberries, glazed, and topped with rainbow sprinkles? I didn't think so. —Joy Huang | The Cooking of Joy
Pulse the flour, sugar, and salt together in a food processor. Add the butter and pulse until the butter has been cut into pea-sized pieces. Add the water and pulse to combine. Add more water if needed, a tablespoon at a time, just until the dough comes together. Divide the dough in two, wrap with plastic wrap, and refrigerate for at least 30 minutes or as long as overnight.
Preheat the oven to 400° F. Tear off two large pieces of parchment paper and tuck each into a 9- by 13-inch baking pan, creasing along the sides and corners so you know how large to roll the dough. Remove the parchment from the pan then, one at a time, roll out the two pieces of dough to about 10- by 14-inch on the parchment paper. It's okay if it's not perfect since you'll be tucking the sides over and under. You can cut off any excess pieces and patch up other corners or cracks if needed, too. Transfer the rolled-out dough, still on the parchment paper, back to the refrigerator while you prepare the filling.
Wash, hull, and thinly slice the strawberries. Toss with the sugar, cornstarch, salt, and lemon juice.
Tuck one of the rolled-out crusts with the parchment paper into the baking pan, making sure there is enough dough on all sides to form a rim/lip. Spread the strawberry filling evenly over the crust. Top with the other rolled out pie crust (you can use the parchment paper to invert the dough sheet over the strawberry filling). Carefully crimp together the edges and fold the excess dough over and under the edge, lifting up the parchment paper underneath to help with this process. If there was a lot of extra dough, you can trim it off, but you want to make sure the filling is sealed inside. Brush with milk or cream, if you like. Use a fork to prick the top crust evenly all around. Bake for 20 minutes.
Check to make sure that the holes you made are still open (sometimes they shrink and seal up); if not, prick again with a fork—this is to ensure steam can exit. Rotate and return to the oven for another 20 minutes, or until golden brown.
In the meantime, make the glaze by mixing together the confectioners' sugar, milk, and vanilla extract. Add more confectioners' sugar if the glaze is too thin, or dribble in some more milk if it's too thick.
Allow the pie to cool before using the parchment paper as a sling to remove the pie from the pan and onto a large cutting board. Spread the glaze over the pie, leaving a 1/2-inch border. Immediately top with the rainbow sprinkles. Slice and serve.