Key Lime Cupcakes

Note (for self-rising flour): Or whisk 11/2 teaspoons baking powder and 1/4 teaspoon salt into 1 1/2 cups bleached all-purpose flour.

Adjust oven rack to middle position; heat oven to 400 degrees. Line a 12-cup muffin tin with cupcake papers. Beat 12 tablespoons butter, sugar, and 1 1/2 teaspoons lime zest until light and fluffy. Starting and ending with an egg, alternate beating in eggs and flour until each is thoroughly incorporated. Beat in lime juice and vanilla until just incorporated. Divide batter evenly among prepared pans. Bake until golden brown, 15 to 17 minutes. Remove from oven, let cupcakes stand a couple of minutes; turn onto a wire rack to cool.

Meanwhile, beat remaining 4 tablespoons butter, 1 1/2 teaspoons zest, and the cream cheese until smooth; beat in condensed milk and sugar. Frost and serve.