Preheat oven to 350° F. Spray a 8 x 8 x 2-inch baking pan with nonstick spray, set aside.
For the Cake
Whisk flour, sugar, baking powder, baking soda, and cinnamon together in a large bowl. Set aside.
In a medium bowl, whisk together the Greek yogurt, vanilla, lemon juice, egg, and oil until well combined. Mix in the mashed banana and stir to combine.
Pour the wet ingredients into the dry ingredients and mix just until well combined. Stir in the chocolate chips.
Pour into the prepared baking dish and smooth the top to even it out. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean. Allow cake to cool completely before frosting.
For the Frosting
Beat peanut butter and butter together until light and smooth with an electric mixer.
Add half of the milk (3 tablespoons) and vanilla and beat to combine. Mix in the powdered sugar and cocoa powder.
Gradually add the remaining milk until fluffy frosting forms.
Spread frosting over the completely cooled cake. Sprinkle sea salt flakes over the top of the cake, if desired, slice and serve.
Notes
Cake can be made up to two days in advance. Cover and store in the refrigerator until ready to use. Allow cake to come up the room temperature for atleast one hour before serving.
Store leftover cake in an airtight container or covered with plastic wrap in the refrigerator for up to 3 days.
Cake can be decorated with chocolate chips and chopped peanuts if you prefer.
To make without frosting: increase sugar to three-quarters cup.
All ovens cook differently, your cooking time may be different than mine.
If using a 9 x 9-inch pan, cooking time may be different.