Read about this cake at Bake or Break.
Preheat oven to 350°. Grease two 9-inch round cake pans.
Whisk together flour, baking powder, salt, and cinnamon. Set aside.
Beat butter, granulated sugar, and brown sugar until fluffy. Add eggs, one at a time, beating well after each addition. Mix in vanilla.
Gradually add flour mixture and mix until combined. Stir in sour cream. Stir in pecans.
Bake 22-25 minutes, or until a toothpick inserted into the center comes out clean. Cool in pans on wire rack.
In a large bowl, mash banana. Stir in butter and lemon juice. Add sugar in 3-4 additions and mix until smooth and spreadable.
Place one cake layer on cake plate. Cover the top with half of the icing. Top with remaining cake layer. Cover the top with remaining icing. Garnish, if desired, with pecans.
Refrigerate cake until frosting has set (1-2 hours) before serving.