In a saucepan, place the sugar and carefully pour the water around the edge. Using your finger, make an X in the pan to help the water gently mix with the sugar. Bring to a boil and cook to softball stage (235 degrees F.)
Meanwhile, whip the egg whites in a mixer with a whip attachment until light and fluffy. When sugar is at the softball stage, slowly pour it into the bowl down the side and continue whipping, adding vanilla, until incorporated and then cooled.
Add the butter 1 cube at a time while whipping until it is a thick butter cream (check flavor and consistency of butter cream before continuing to add butter, as not all of the 2 cups may be needed).
Keep at room temperature while frosting the cake.