Make the filling: in a medium saucepan toss together the berries, lemon juice, sugar and corn starch. Cook, stirring, over medium heat for 4-5 minutes or until the berries break down and the sauce thickens slightly. Let it cool completely.

Make the pastry - in the bowl of a food processor, combine the flour, salt and sugar. Add the cold butter and pulse until the mixture looks like coarse crumbs. Add 4 tablespoons of ice cold water and pulse until the mixture is evenly moistened and starts to come together.

If it is still really dry add the remaining 1 tablespoon of water. Divide the pastry in half and form each half into a disc. Wrap each disc tightly in plastic wrap and refrigerate for at least one hour.

On a lightly floured surface, roll out one piece of the chilled pastry until it is very thin, and cut out nine 3x5 inch rectangles (there will be just enough pastry for 9 pieces, you will have to re-roll the scraps once or twice to get them all).

Lay the rectangles on a parchment lined baking sheet. Spoon 1 1/2 tablespoons of the berry filling onto each rectangle, leaving the edges bare. Roll out the other half of the pastry and cut out the top rectangles, laying them over top of the filling.

Seal the edges of the pop-tarts with a fork, and poke several holes in the tops with a toothpick to vent. Place the baking sheet in the freezer for 1-2 hours to chill.

Preheat the oven to 375°F. Bake the pop-tarts for 25-30 minutes. Let them cool on the baking sheet for 10 minutes and then carefully move them to a wire rack to cool completely.

Whisk together the powdered sugar, milk and vanilla until the frosting is the desired consistency (start with 1 tablespoon of milk, add more if needed) and spoon the frosting onto the cooled pop tarts. Top with sprinkles if desired.