Preheat oven to 350°F.
Prepare loaf pans. Spray pans with nonstick spray and set aside.
With electric mixer, using the whisk attachment, cream together the butter, brown sugar, granulated sugar and eggs. Cream about 2 minutes, until fluffy.
Add the vanilla, sour cream, milk, mashed bananas and sourdough discard. Mix well.
In a separate bowl, mix the flour, baking soda and salt with a whisk.
Add the flour mixture to the wet ingredients and mix on medium low speed until fully combined, about 1-2 minutes
Pour batter into pans and fill 2/3 full.
Bake loaves on center rack for 40-50 minutes for mini loaves (5.75 x 3.25 x .25-inch pan) and 50-55 minutes for standard size loaves (8.5 x 4.5 x 2.5- inch pan). Insert toothpick or cake tester in the center of the loaf. If it comes out clean, they are ready to take out. If not, bake another few minutes and check.
Allow the banana bread to cool in the pans for 10 minutes. Turn them out of the pan and place on wire rack to cool completely (or mostly) before slicing. Enjoy!