Preheat your oven to 350°F (175°C) and line a cupcake pan with paper liners.
Cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the grapefruit juice and zest.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk.
Mix until just combined, then stir in the vanilla extract.
Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
For the frosting, beat the softened butter in a bowl until creamy.
Gradually add the powdered sugar, mixing until smooth.
Add the grapefruit juice, zest, vanilla extract, and a pinch of salt, then beat until fluffy.
Once the cupcakes are completely cool, frost them generously with the grapefruit buttercream.