To make the dough, combine the all-purpose flour, white whole wheat flour, and kosher salt in a large bowl. Cut in the cold salted butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. This is the beginning of your flaky crust!
Add cold water, starting with about 1/4 cup, one tablespoon at a time, mixing until the dough just comes together. You may need to adjust the amount depending on humidity. It should be slightly tacky but not sticky.
Once the dough is ready, divide it into two equal parts, wrap them in plastic wrap, and refrigerate for at least one hour or freeze for 20 minutes. This resting time makes for a better texture.
After the dough has chilled, preheat your oven to 400° F. Flour your workspace lightly and roll out one half of the dough into a rectangle about 1/4-inch thick. Aim for dimensions close to 8×16 inches.
Transfer this rolled-out dough to a parchment-lined baking sheet. Spread the strawberry jam evenly across the surface, leaving a 1-inch border around the edges.
Now, roll out the second half of your dough in the same way, and carefully lay it over the jam-covered layer. Sealing this pop-tart is crucial, so go ahead and crimp the edges with the back of a fork to ensure no filling escapes during baking.
Cover the baking sheet with plastic wrap and let it chill in the fridge for another 30 minutes. This step helps prevent the dough from puffing up too much in the oven.
Before baking, brush the top with the beaten egg for that gorgeous golden finish. Pop it into the preheated oven and bake for 30 to 35 minutes or until the crust turns a beautiful golden brown.
Once done, let the giant pop-tart cool for about 10 minutes on the sheet before transferring it to a wire rack.
While it’s cooling, whisk together the powdered sugar, vanilla extract, and enough hot milk to achieve a smooth frosting consistency. Spread this over the cooled pop-tart and let it set for a few minutes.
Finally, sprinkle on some fun sprinkles for that festive touch. Let everything set for an hour or overnight, then slice into 9 generous rectangles. Serve and enjoy your delicious creation!
