Adapted from Sunset
Add 3/4 cup flour, 3/4 cup brown sugar, and 1/4 teaspoon salt to the bowl of a food processor.
Cut 1/4 cup butter into chunks and add to the flour mixture.
Pulse the mixture in the food processor until it resembles crumbs. Set it aside.
Beat 1/4 cup butter and 1 cup brown sugar together in a large bowl until smooth.
Add mashed bananas and mix.
Whisk together the 1 1/2 cups flour, baking powder and salt.
Stir the dry ingredients into the wet ingredients.
Fold in the sour cream gently until just incorporated.
Divide batter into a muffin pan.
Sprinkle streusel on top of muffins.
Bake at 350 degrees for 18 - 20 minutes or until toohepick comes out clean.
Cool for about 5 minutes in the pan and than remove to cool completely on a cooling rack.