Preheat the oven to 350°F and grease a 9×13 inch pan.  I've also made this in two 8-inch rounds (plus a little ramekin for my own pleasure).

To make the cake, mash the bananas in a bowl and set aside. In a stand mixer (paddle attachment) or with a handheld electric mixer, cream the butter and sugars together until well-combined and fluffy (about 2 minutes).

Add the eggs and vanilla and blend together, scraping down the sides of the bowl as necessary.  Then add the mashed bananas and mix together.

With the mixer on low, add the dry ingredients (flour, baking powder, baking soda, cinnamon, and salt) along with the buttermilk in three additions, alternating between the wet and dry.  Mix until *just* incorporated---don't overmix!  A few lumps are totally fine.

Add the peanuts if you're using them, and give a couple light pulses to stir them in.

Spread the batter into your prepared pan(s) and bake for 40-50 minutes; if you're doing two smaller round pans it will be more like 30-35.  Baking times will depend on your oven, so keep an eye on yours and if it starts to brown too soon you can loosely cover it with aluminum foil.  When it's done, a toothpick inserted should come out clean with a few crumbs.

Remove the cake from the oven and set it on a wire rack, letting it cool completely.

To make the frosting, beat the butter in your stand mixer or with an electric hand mixer, then add the peanut butter and beat for about 30 seconds to combine it.  Add the powdered sugar and a pinch of salt and beat until fluffy.  Add 1 tablespoon of the cream or milk, as needed to get the right texture.  I often add some more sea salt for a nice salty balance.

Frost the cake, and either serve immediately or cover.  You can store leftover cake in the refrigerator for up to 5 days.