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1
Make the candied zest: Remove the grapefruit zest with a vegetable peeler and slice into thin strands.
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2
Place in a saucepan; cover with water and bring to a simmer.
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3
Drain the zest and return to the saucepan; cover with water and repeat 2 more times.
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4
Set the zest aside.
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5
Juice the grapefruit into the saucepan, reserving 1 tablespoon for the frosting.
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6
Add the liqueur and granulated sugar; bring to a simmer over medium-high heat, stirring to dissolve the sugar.
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7
Add the zest, reduce the heat to low and simmer 20 minutes.
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8
Remove with a slotted spoon; set aside.
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9
Reserve the syrup.
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10
Make the cupcakes: Preheat the oven to 350 degrees F and line a muffin pan with 12 cupcake liners.
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11
Whisk the flour, baking powder and salt in a medium bowl.
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12
Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 5 minutes.
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13
Beat in the eggs, one at a time, then beat in the vanilla and the grated grapefruit zest.
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14
Reduce the speed to medium and add the flour mixture in 3 additions, alternating with the milk, beginning and ending with the flour mixture; beat until the batter just comes together.
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15
Divide the batter among the muffin cups, filling each two-thirds of the way.
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16
Bake until a toothpick inserted into the centers comes out clean, 18 minutes.
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17
Poke a few holes in each cupcake with a toothpick.
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18
Reheat the grapefruit syrup; brush generously over the cupcakes (reserve the remaining syrup).
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19
Remove the cupcakes to a rack to cool completely.
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20
Make the frosting: Beat the butter in a large bowl with a mixer on medium-high speed until fluffy, about 2 minutes.
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21
Add the confectioners' sugar, food coloring, grated grapefruit zest, liqueur and the reserved 1 tablespoon grapefruit juice; beat until smooth, about 3 minutes.
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22
Spread the frosting on the cupcakes.
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23
Dip grapefruit segments in the remaining syrup and set on the cupcakes; top with the candied zest.
