Pre-heat the oven to 375°F.

Line the muffin cups with cupcake liners (or grease the muffin cups well if not using liners).

Cream (or mix) the butter and sugar in a large bowl until you have a light and fluffy mixture.

Then beat in the egg, and blend in the milk.

Add in the baking powder, salt, and flour, mixing well.

Gently fold in the chopped strawberries and blackberries by hand.

Lastly, pour or scoop the batter into the muffin cups. I used this batter spoon to make this step easier.*

Bake the muffins at 375°F for about 25 to 30 minutes or until the tops are golden and a cake tester or toothpick comes out clean.

Allow the muffins to cool in the muffin tin for about 10 minutes. Then remove them to a wire rack to cool completely.