Freeze the butter.

Preheat oven to 400 degrees Fahrenheit.

Grate the frozen butter on a coarse grater.

Use a pastry cutter or food processor to mix the flour, salt, and butter until it is crumbly. You want small crumbs.

Mix in the egg and stir until well combined. If the dough does not gather into a ball, add cold water (1-2 tablespoons) one spoonful at a time.

Place the frozen berries in a saucepan. Pour in the sugar and starch.

Bring the contents of the pot to a boil stirring frequently.

Cover with a lid and allow to cool.

Prepare a 9" x 9" baking dish by greasing it with butter on the bottom and edges. Sprinkle with flour and shake out any extra flour.

Use a rolling pin to roll the dough into a 1/4-inch thick sheet. Place the dough in the pan, seal the corners, and cut off the edges. Prick the dough with a fork.

Set aside the rest of the dough for the decorations.

Place the filling on top of the crust and smooth into an even layer.

Bake the tart in a 400 degree Fahrenheit oven for 20 minutes.

Cool the tart in the pan.

After it's cooled, run a thin knife between the walls of the pan and the tart and carefully remove the tart from the mold.

Roll the remaining dough out to 1/4-inch.

Cut four 8-inch-long strips and place them on parchment paper in a grid pattern to form the tic tac toe grid.

Cut out hearts with a heart cookie cutter, and form Xs by making strips of dough and criss crossing them.

Sprinkle sugar over the tic tac toe grid and Xs and Os.

Bake until browned, 8 to 10 minutes.

Cool and transfer it to the pie carefully or it could crack.

Cut the tart into squares, decorate with marshmallows if you'd like and serve.