Preheat oven to 355°F (180°C). In a medium-sized bowl, beat butter and sugar until pale and fluffy. Add the egg, bananas, and vanilla, and beat for 1 minute.
Sift over the flour, baking powder, soda, and salt. Beat until just combined, stir in the chopped strawberries, and/or raisins.
Pour the batter into the greased muffin pan and bake for 18-20 minutes or until a toothpick comes out clean. Cool for 5 minutes then transfer to a wire rack to cool completely.
Notes
Store in an airtight container, or freeze in a rigid container or sealed plastic bag.