Preheat oven to 350 degrees F. Line muffin tins with paper cupcake liners.
Cream the butter, cream cheese and sugar at medium speed, then reduce to low. Add the eggs one at a time, then add the grapefruit juice and vanilla extract.
Sift together the flour, baking powder, and salt. Add the grapefruit zest to the flour mixture.
Incorporate the dry ingredients gradually into the wet ingredients, mixing until just combined. Do not over mix.
Scoop batter into lined muffin tins with an ice cream scoop (about 3/4 full). Bake about 20 minutes, rotating pans halfway through.
Cool for 5 minutes before removing from pan, then move to racks to cool completely.
For frosting: Beat the butter until smooth and add the vanilla extract. Slowly add the powdered sugar, then the grapefruit juice and zest. Add a little milk if frosting is too thick or not smooth. Add more powdered sugar if the frosting is too thin.
Frost the cupcakes once they reach room temperature.