Preheat the oven to 350 F (177 C). Lightly grease and line the bottom and two long sides of a 13 x 9 pan.
In a mixing bowl, whisk the flour, cocoa powder, baking soda, baking powder and salt together until well combined.
1¾ cups (210 g) all purpose flour, ½ cup (40 g) cocoa powder, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon salt
Whisk the bananas until smooth with just a few lumps. Whisk in the oil, granulated sugar, light brown sugar, eggs and sour cream together until smooth and well combined. 1 cup mashed bananas , ¾ cup (177 g) oil or melted butter, 1 cup (200 g) light brown sugar, ¼ cup (50 g) granulated sugar, 3 large eggs (US), room temperature, ½ cup (120 g) sour cream, room temperature
Whisk the dry ingredients into the wet well combined with no visible pieces of flour. I like to whisk ½ of the dry ingredients at a time.
Pour the chocolate banana cake batter into the prepared pan. Use a flexible spatula to smooth the batter evenly across the pan. Bake for about 28 - 32 minutes or until the banana cake bounces back when gently pressed. The edges of the chocolate cake will also pull slightly away from the pan. Cool the cake in the pan for 15 minutes. Use the parchment paper on the two long sides of the pan to gently lift the cake out of the pan and onto a cooling rack.
Cream together room temperature butter with the room temperature cream cheese until creamy and well combined, about 4-5 minutes on low speed.
½ cup (113 g) unsalted butter, room temperature, 8 oz. (226 g) cream cheese, room temperature
Mix in the peanut butter, vanilla extract and salt until smooth and well combined. Mix in powdered sugar ½ cup (60 g) at a time. Once all the sugar has been added cream the peanut butter icing on high until the frosting is your desired consistency.
½ cup (128 g) creamy peanut butter, 2 teaspoons (10 ml) vanilla extract , ½ teaspoon salt, 4 cups (480 g) powdered sugar
Spread the frosting on top of the fully cooled chocolate and banana cake. Make the swirls using the back of a small spoon. Top with shaved chocolate if you like. Slice and enjoy!