Preheat oven to 350℉ (180℃)
Prepare an 8-inch round cake pan by coating with baking spray and lining the bottom with parchment paper.
Slice one banana into ¼" slices. Mash second banana until no lumps remain.
2 Large Bananas, ripe
Place chocolate and butter in a heat-resistant bowl. Microwave on 50% power for 30 seconds and stir briefly. Continue to heat at 30 second intervals, stirring between each heating, until butter and chocolate are melted.
1 Cup Chocolate chips, bittersweet, 8 Tablespoons Unsalted butter, softened
Add sugar, salt and vanilla to the bowl with the melted chocolate. Whisk until well blended.
¾ Cup Granulated sugar, ½ teaspoon Salt, 2 teaspoons Pure Vanilla Extract
Add the eggs and mashed banana to the mixing bowl. Whisk until well blended.
3 Large Eggs, room temperature
Add the cocoa powder to the mixing bowl. Whisk until well blended.
½ Cup Dutch-processed cocoa powder, sifted
Stir in toasted walnut pieces.
½ Cup Walnuts, toasted and chopped
Pour half of the cake batter into the prepared baking pan. Place a layer of the banana slices over the top of the batter.
Pour the remaining cake batter into the pan over the top of the banana slices. Smooth gently with an offset spatula.
Bake for 30 minutes in pre-heated oven. The cake is finished when the internal temperature of the cake is 200℉ (use an instant read thermometer inserted into the center of the cake to check for doneness).
Place the cake on a wire rack for 10 minutes. Run an offset spatula around the inside of the cake pan and flip over onto a wire rack. Let the cake cool completely