While cupcakes bake, beat butter with paddle attachment of mixer until very smooth. Add powdered sugar and 2 teaspoons of lime zest; beat on slow speed. While mixer is running, mix together in a liquid measuring cup, 1/4 cup lime juice, 1 tablespoon tequila and 1 teaspoon Grand Marnier. Add 4 tablespoons of liquid mixture to the butter/powdered sugar mixture (1 Tablespoon at a time,) beating well between each addition. Once all 4 tablespoons are added, beat on medium-high speed for about 5 minutes. Then let your mixer run at low speed until you are ready to use the frosting. This keeps the frosting from getting crusty.