Beat butter and sugar together.
Add eggs one at a time.
Add lemonade concentrate and vanilla.
Mix flour, baking powder, baking soda, and salt in a small bowl.
Add 1/3 of the flour mixture.
Add 1/2 of the buttermilk.
Repeat until all of the buttermilk and flour is incorporated.
Pour into a 9×13 that has been coated with cooking spray. Bake in a 350 degree oven for 30 to 35 minutes. Cool completely.
Beat the butter until smooth. Add 1 cup of sugar. Add in 1/3 cup of the lemonade concentrate and vanilla.
Gradually add the rest of the powdered sugar. Add remaining lemonade until you get a nice consistency.
Spread frosting over cooked cake.