In a large bowl, whisk together butter, salt, brown sugar, granulated sugar, molasses, and vanilla until combined. Add mashed banana and whisk again. Add eggs and whisk until incorporated, then buttermilk. Sprinkle baking soda and baking powder on batter and whisk until combined and then (don’t skip this), 10 to 20 more times, ensuring that it very well dispersed in the bowl. Add flour and mix only until it disappears. Divide batter between 12 cups.