Giant Frosted Strawberry Pop Tart

1. To make the dough. In a bowl, combine the whole wheat flour, flour, and salt. Add the butter and toss with the flour until the mix clumps together. Add 1/4-1/3 cup cold water, 1 tablespoon at a time, until the dough forms a ball. Or use a food processor. 
2. Turn the dough out onto a floured surface and divide in half. Roll each half out into a 1/4-inch thick rectangle, about 8x16 inches. Transfer 1 sheet to a parchment-lined baking sheet and evenly spread on the jam, leaving a 1-inch border around the edge. Lay the other half of the dough over the filling and seal the edges by crimping with the back of a fork.  Cover the baking sheets and place in the fridge for 1 hour or in the freezer for 20 minutes.
5. Preheat the oven to 400° F. Brush the pop-tart with the beaten egg and bake for 30-35 minutes, until the crust is golden brown. Let cool.
6. Meanwhile, in a medium bowl, whisk together the powder sugar, vanilla, and 2 tablespoons hot milk, adding the milk to thin the frosting as desired. Spread the frosting over the pop-tart. Let the frosting set a few minutes, then decorate with sprinkles. Let sit 1 hour or overnight to allow the frosting to dry. Cut the tart into 9 rectangles. Serve or store in an airtight container for up to 3 days at room temperature.