Preheat oven to 350 degrees F. Spray or oil a 11 1/2X8 (for thicker cake) or a 9X13 (for thin cake), set aside.
In one bowl, combine the cake flour, baking powder and salt. Stir to combine, set aside.
In another bowl, cream together the sugar, eggs, oil and vanilla until creamy. Add in the lemon(or lime) zest, if using. While the mixer is running, alternate adding some flour, then some milk until batter is smooth.
Pour into prepared pan and bake for 20 to 25 minutes or until knife comes out clean from center of cake. Start checking the cake after 20 minutes. If you over bake, it will be dry. Remove from oven and let cool.
Once cake has cooled, prepare frosting. In a large bowl, whisk together the powdered sugar, butter, milk and vanilla. Add the food coloring and stir until well combined. If using zest, mix that in . Frost the cake in the pan and add sprinkles. The frosting will firm up as it sets.