Preheat the oven to 350° and grease a 9x13 baking pan.
Combine the flour, baking soda, and kosher salt. Set aside.
Cream together the butter, sugar and brown sugar. Add in the eggs, vanilla, buttermilk and mashed bananas. Mix until it is combined.
Slowly add the flour mixture to the batter. Mix just until it is combined.
Pour the batter in the prepared baking pan.
Bake 25-30 minutes. Test that the middle is baked through by poking with a toothpick, if it comes out clean or with cake crumbs it is ready, if you have batter on the toothpick, loosely cover the cake with foil and bake another 5 minutes.(if you are using a dark metal baking pan, it will be done closer to 25 minutes)
Cool completely and frost with cream cheese icing. Frosting Tip: Start with half the frosting, doing a "crumb coat", then finish with the remaining frosting for a pretty top layer. To make swirls in your frosting use the back of a spoon.
Cream Cheese Frosting
In a stand mixer with a paddle attachment (or bowl with hand held mixer), combine the cream cheese, butter, vanilla and half the powdered sugar. Mix until it comes together.
Add the remaining powdered sugar and continue to mix until it is creamy and smooth.
Storing
Keep banana bars at room temp in an airtight container for up to 4 days.
Notes
RECIPE TIPS:
Make sure your bananas are totally ripe and will mash down to almost liquid.
Don't skip the buttermilk, you need the acid to activate the baking soda.
Sift the powdered sugar when you make the cream cheese icing, it will make for smoother consistency.
Make sure your butter and cream cheese are softened to the core so you don't end up with lumpy frosting!
To quickly soften bananas separate them from the group stem and place in a paper bag with one ripe apple or banana. OR place unpeeled bananas on a foil lined baking sheet in the oven at 300 degrees for 20-30 minutes.