Preheat oven to 350
Line cupcake trays with liners.
Beat butter and sugar in a large bowl for 3 minutes until light and fluffy.
Add in lemon juice, zest, and vanilla extract, beat for one more minute until combined.
In a separate medium bowl, sift cake flour, baking powder and salt.
In another small bowl, use a whisk to mix together the egg whites, oil and buttermilk.
Slowly add a little of the flour mixture to the butter mixture, add a little of the buttermilk mixture. Switch off between adding the flour and buttermilk mixture, ending with the buttermilk mixture.
Beat for 2 minutes to combine all the ingredients thoroughly.
Fill the liners ¾ of the way and bake for 20 minutes.
Add all of the ingredients into a stand up mixing bowl and mix until combined.
If the frosting is too thick add in 1 tablespoon of heavy whipping cream.
If the frosting is too thin add in powdered sugar, ¼ cup at a time.
Pipe onto cupcakes.