Preheat oven to 350 degrees F. Line 2 regular muffin tins with cupcake liners.
In bowl of stand mixer, stir together dry ingredients.
Add eggs to dry ingredients, beat until mixture is completely incorporated and smooth.
With mixer running on low speed, slowly add rum, grapefruit juice, zest, and vanilla. Turn speed up a little, mix until well incorporated and smooth.
Carefully add melted butter, and food coloring if you are using it. Continue running the mixer until the butter is completely incorporated, scraping down the sides of the bowl after a minute or so. Batter should be very smooth.
Pour into prepared cupcake pans – I like to use an icecream or cookie scoop.
Bake until fully golden and tester comes out clean and cake springs back – about 20 minutes. Cool completely before frosting.
Prepare Swiss Meringue Buttercream, adding rum, grapefruit juice, and zest AFTER the butter has been beat in, and the buttercream has ‘come together’.