In a 12-inch cast iron skillet, melt butter over medium heat.
Add brown sugar, peanut butter, and water, whisking until smooth.
Bring to a boil, and cook for 2 minutes.
Remove from heat, and add vanilla.
Stir in banana slices, arranging them in a single layer.
In a large mixing bowl, beat butter and sugar until creamy.
Add egg, vanilla, and peanut butter, beating until combined.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the mixing bowl, beating on low speed, alternating with the whole milk.
Beat until just combined.
Working from the outer edges to the center, spoon cake batter over the bananas in the skillet. It’s a very thick batter, so you’ll drop it by spoonfuls or dollops, and then use a spatula to carefully spread and cover the bananas completely.
Bake for 40-45 minutes, or until the edges and center appear set, or until a toothpick inserted near the center comes out clean of cake crumbs.
Remove from heat.
Cool for 20-30 minutes before inverting onto a serving platter.