Preheat oven to 400F. Lightly spray a mini 24 cup muffin pan with the cooking spray.
Whisk the flour and baking powder together in a large bowl making a well in the center.
Whisk the egg and milk together and pour into the well. Add the tomato, bocconcini, basil and butter. Fold until just combined.
Pour the mixture into 24 muffin cups dividing evenly. Bake for 25 min. or until golden brown. Cool 5 min. then pop them out to cool completely on a cookie rack.