In a large bowl or bowl of a stand mixer, cream together butter and brown sugar. Add bananas and mix, then mix in eggs.
8 tablespoons unsalted butter, 1 cup brown sugar, 4 bananas, 2 large eggs
Gradually add dry ingredients to banana mixture, mixing just until fully incorporated and no dry bits remain.
Pour batter into prepared loaf pan, spreading to the edges and corners. Bake for about 60 minutes, until golden and a toothpick inserted in the center only has moist crumbs. Let cool in the pan a few minutes before removing to a wire rack to cool completely.
Notes
Your bananas should be very ripe with lots of brown spots on the skin and mash easily.
If your bananas are still yellow, you can set them on a foil-lined baking sheet and bake at 300°F for 10-15 minutes, until the skins are fully browned/black. Let cool before using.