Sift cocoa, flour, sugar, bicarb soda, baking powder, and salt into the bowl of an electric mixer.
Beat on low speed until just combined. Raise speed to medium and add eggs, milk, water, oil, and vanilla. Beat until smooth, around 3 minutes.
Divide batter between pans. Bake for approximately 35 minutes, until a cake tester comes out clean. Cool in pan for 15 minutes then turn onto wire rack to cool completely.
Frosting:Place the butter in a large mixing bowl and on medium speed of an electric mixer beat until smooth. Add the sugar, 2 tablespoons of milk, and vanilla. Beat until smooth and creamy, about 5-7 minutes. Gradually add the remaining milk or until the icing is thick enough to be of good spreading consistency. Add sifted cocoa and mix thoroughly.
Sandwich the two cake layers together with frosting. Cover top and sides of cake with remaining frosting. Stand Kit Kats around outside of cake. We used three family sized blocks of Kit Kat Chocolate. Cover top of cake with M & M's.
Recipe by A Spoonful of Sugar at https://aspoonfulofsugardesigns.com/kit-kat-birthday-cake/