Preheat oven to 350°F. Line a cupcake pan with cupcake liners.
Combine flour, baking powder and salt in a medium bowl, set aside.
Cream butter and sugar. Then add eggs and vanilla, making sure they are fully incorporated.
Mix flour mixture into butter mixture until just combined.
Add in buttermilk, grapefruit juice, and lemon zest. Mix until fully combined.
Divide batter into 12 cupcakes.
Bake for 18-20 minutes or until a toothpick inserted comes out clean.
Let the cupcakes cool completely before frosting.