In a food processor, place the sugar and pulse until super fine, 2 minutes. Sift half of the sugar with the flour and salt, set aside.

In a large bowl with hand mixer or stand mixer, place the egg whites, add in the water, vanilla and cream of tartar. Mix until fully combined. Gradually add in the remaining sugar until incorporated. place mixer on medium speed until egg whites form medium peaks.

Start to add in the flour mixture a little at a time by sifting it onto the surface of the egg mixture and folding it in in very small batches. Finally, add in the lemon and lime zest and fold in.

Fill the cupcake liners almost all the way to the top. Bake for 16 to 18 minutes or until a toothpick inserted comes out clean. Let cool for 5 minutes in the tin, then transfer to a wire rack to cool completely.

Using a hand mixer, cream butter in a large bowl. Add in the powdered sugar a little at a time, this will form a thick paste. Add in the vanilla and milk, mix to combine. Add in the lemon and lime zest and half of the whipped topping and whisk in on low speed until smooth. Take the remaining whipped topping and fold in with a spoon or spatula. Place in fridge for 15 minutes to set up.

Frost cupcakes, garnish with more lemon and lime zest if desired, enjoy! These cupcakes must be refrigerated if not eaten right away.