Spray 2 7-inch cake pans with cooking spray and line them with parchment paper. (Watch the video for how I do this.)
Add the flour, sugar, baking powder, baking soda, and salt into a large bowl. Whisk and set aside.
To a small saucepot, add cocoa powder, vegetable oil, water, peppermint extract, espresso, vanilla extract, and banana. Stir together over medium-low heat until the cocoa powder has completely dissolved.
Stir the warm chocolate mixture into the dry ingredients. Using a whisk, mix together all ingredients until the dry ingredients are completely incorporated.
Divide cake batter equally into the two cake pans (use a scale--it's not as intuitive as you think!).
Bake in the oven for ~38 minutes, until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for about 5 to 10 minutes. Run a sharp knife or offset spatula around the edges to loose the cakes before flipping the cake pans over and gently dislodging the cakes.
Allow the cakes to cool on a wire rack for at least 1 hour before frosting them. You can also wrap them up and place them in the fridge overnight (this will actually make it much easier to frost them).
Chocolate Buttercream Frosting
Add powdered sugar and cocoa powder into a large bowl.
Add 1/2 of the butter, melted chocolate and vanilla extract and mix using handmixer until the mixture becomes creamy. If the frosting is too thick, additional butter (1 tablespoon at a time) and plant milk until you reach desired consistency.
Wait until the cakes have completely cooled before attempting to frost them. I generally do not like too much frosting, but if you like frosting, double the frosting recipe.
If you don't have espresso handy, you can substitute in 2 teaspoons of instant coffee powder, but add 1/4 cup (for a full 2 cups) of water to the recipe.