To make the filling, dissolve flour into lemon juice. Add berries, sugar, combine all ingredients in a small saucepan. Cook on medium heat for 10 minutes until thickened and reduced.
Preheat the oven to 350°F.
Remove the dough from the refrigerator and divide it in half. Press each half into a rectangle. On a lightly floured surface, roll out each half into a 14-by-11-inch rectangle. Using a paring knife, lightly score 1 rectangle into eight 3-1/2-by-5-1/2-inch rectangles (about the size of an index card). Using a knife, pizza roller or a fluted pastry roller and following the scored lines, cut the dough into 8 rectangles. Repeat with the other half of dough.
Place 8 rectangles, well-spaced, on parchment lined baking sheet.
Brush the top surface of the entire rectangle with the egg. Spoon 2 Tbs. of berries in a mound in the center of each rectangle. Lay the second dough rectangle directly on top of the first and use fork or pastry wheel to seal around each fruit pocket, so the pastry sheets adhere to each other.
Freeze for 15 minutes.
Bake for 20-25 minutes, or until the tops of the pastries are evenly golden brown. Let cool on the baking sheet on a wire rack for about 30 minutes.
For the icing:
Whisk together the powdered sugar, milk, and vanilla. Brush top of pop tarts evenly with icing and then sprinkle with colored sprinkles. Set for 10-15 minutes before serving. The pastries can be stored in an airtight container at room temperature for up to 2 days.