Preheat oven to 400° F.
In a small mixing bowl, combine egg, milk, oil, sour cream, and mashed banana.
In a separate medium mixing bowl, combine flour, sugar, baking powder, and salt.
Add wet ingredients to dry ingredients and mix just until moistened.
Spoon batter into greased muffin cups, filling each 3/4 full.
Top each with a banana slice.
Bake at 400° for 20 minutes or until golden brown.
Remove from oven and cool on wire rack for 5 minutes.
Remove from muffin cups.