Heat oven to 375°F. Grease or line 12 (2 1/2 inch) muffin cups with paper baking cups. Combine butter and sugar, beat until well blended. Add egg and blend well. Stir in banana and vanilla. Combine flour, baking powder, baking soda, salt and cinnamon. Add to butter mixture; stir just until dry ingredients are moistened. Fold in prunes. Spoon batter into prepared muffin cups. Bake in upper third of oven for 20 to 25 minutes or until toothpick inserted in center comes out clean.