Preheat oven to 375˚F. Grease muffin pan, or line with parchment liners.
Add vinegar to measuring cup, then add enough milk to reach the 1 cup mark to sour the milk. Let sit for about 5 minutes to soften the bran.
Add bran to the sour milk.
In a large bowl, mash very ripe bananas. Add egg and greek yogurt. Whisk together well.
Add bran mixture to banana mixture.
Add flour, baking soda, baking powder, and salt. Whisk these dry ingredients a bit, then mix everything together until just combined, don't over mix.
Fold in raspberries gently. Drop into prepared muffin pan.
Bake for 15-20 minutes until brown and a toothpick comes out clean when inserted into the middle of the muffin.