CAKE:
9 ounces Margarita Mixer, bottled liquid
3 ounces tequila
3/4 ounce Grand Marnier
1 (18.25 ounce) box white cake mix
3 large egg whites
2 Tablespoons canola oil
1 Tablespoon lime zest

LIME BUTTERCREAM ICING:
1 cup unsalted butter, at room temperature
5 to 6 cups powdered sugar
2 Tablespoons lime juice, freshly squeezed
1/8 teaspoon salt
1 teaspoon lime zest
green food coloring, if desired
small lime slices for garnish, if desired