Preheat oven to 375. Cream the butter and sugar until well combined. Add the vanilla, peanut butter, bananas, and eggs. Cream until just combined.
In a seperate bowl, combine the flour, oats, baking soda, and cinnamon. Add the buttermilk and flour mixture alternately to the butter/banana mixture. Stir gently with a wooden spoon or electric beater on low speed until JUST combined, adding the chocolate chips at the end. Do not overmix.
For the streusel, combine the flour, sugar, and cinnamon in a small bowl. Cut the butter into small cubes. Add the butter cubes and water to the flour mixture and work the mixture with your fingers, pressing the butter into the dry ingredients creating small crumbs.
Spoon into greased muffin tin, filling each cup about ¾ full with batter. Top with a generous spoonful of the streusel crumbs. Bake for 11-15 minutes or until tops are just slightly golden brown and bounce back lightly when pressed.
You could omit the oats and just add a few tablespoons of flour depending on your texture preference and how thick your batter is. Letting the batter sit overnight in the fridge will also allow the oats to get soft – when I did this, I hardly noticed their texture in the batter.