In a mug or microwave safe glass cup (that holds at least 1½ cups of liquid) combine the berries, sugar and cornstarch. Fold with a spoon until everything is covered in the sugar/cornstarch mixture. Microwave 1 minute.
Meanwhile, in a small bowl, combine the flour baking powder and sugar. Stir. Add the melted butter, milk and vanilla. Whisk until only small lumps remain. Pour mixture over the fruit and microwave for an additional 2 minutes (the cake will puff up and be cooked through.) Eat warm with a scoop of vanilla bean ice cream if you have it.
Notes
*You can eliminate microwaving the fruit separately and immediately pour the cake batter on top; microwave 2 minutes instead of 3 minutes total. The only difference is that the cake batter drips down between the fruit so the fruit is more within the cake instead of below it.