The tart, terrific taste of key lime flavors in a light and fluffy buttercream make this a new flavor of frosting that goes well on any tropical baked treat. Try it on banana cupcakes, key lime macarons, or coconut cake or cupcakes to add a little taste of vacation to all your desserts!
In a large mixing bowl, beat butter and shortening with an electric mixer until light and fluffy, about 1 minute. Beat in vanilla extract.
Gradually add powdered sugar, one cup at a time. Start by mixing on low speed, then increase speed to medium and beat well. Scrape sides and bottom of bowl often. When all sugar has been added, frosting will appear dry.
Add key lime juice; beat on medium speed until light and fluffy. Add key lime zest and mix until blended and smooth.