Preheat oven to 375 degrees. Grease a 12-cup standard muffin pan.
In a small bowl, beat together cream cheese, sugar, and egg. Put bowl in refrigerator while you make the muffin batter.
In a large bowl, whisk together flour, baking powder and soda, salt, and sugar; set aside.
In a medium bowl, whisk together bananas, buttermilk, oil, eggs, and vanilla.
Pour wet ingredients into the large bowl filled with flour mixture then stir with wooden spoon just until the mixture is combined.
Spoon enough batter into muffins cups so that they are just under halfway full. Add one spoonful of cream cheese mixture into each cup, then top with remaining muffin batter. Muffin cups will end up being almost completely full.
Bake for 20-25 minutes at 375 degrees. Check after 20 minutes, muffins are done when a toothpick inserted into center comes out clean.