Meanwhile, cook the grape juice in a 2- to 4- cup glass measuring cup on in a microwave oven on High powder, taking care that the juice doesn’t boil over, and boil until the juice is reduced by half, without about 5 minutes. (Cooked in a microwave, the juice will reduce without caramelizing. You can also reduce the juice in a saucepan over medium-high heat, but be careful that it doesn’t burn. Or skip this step altogether, and add 1/4 cup water or bottled white or purple grape juice to the filling.)