Sift flour, cornstarch, baking powder, and salt. Beat butter and sugar and mix banana, buttermilk, and eggs. Add the dry ingredients. Bake in cupcake liners for 15 minutes.
Lemon and blueberry cupcakes
Whisk flour, baking powder, and salt. Beat butter and sugar, add eggs and vanilla. Mix flour and buttermilk. Add lemon juice, zest, and blueberries. Bake for 18 minutes.
Strawberry and Nutella cupcake
Bake vanilla cupcakes and cool them. Scoop out the top and pipe in Nutella, then put the top back. Grind dried strawberries and mix in buttercream frosting. Pipe it over.
Coffee cupcakes
Whisk flour, espresso powder, baking powder, and salt. Beat butter and sugar. Add egg whites, vanilla extract, and sour cream. Mix in dry ingredients and coffee. Bake for 12 minutes.
Marble cupcakes
Beat butter, sugar, eggs, milk, and vanilla. Mix flour, baking powder, and salt. In one part of batter mix cocoa powder. Add both batters alternatively to cupcake liners and bake.