Preheat the oven to 170C/325F/Gas 3. Line a 12-cup muffin tin with muffin cases, and set aside.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Whisk together the eggs. Add the sugar, and whisk until combined. Add the butter, yogurt, lime juice, zest, and tequila, and whisk until combined. Fold in the dry ingredients.
Divide the batter evenly between the muffin cases, filling only 2/3 full.
Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool for 5 minutes in the pan, before transferring them to a wire rack to cool completely before frosting.
For the Frosting
Using a handheld or stand mixer fitted with the paddle attachment, beat the butter on high until pale and creamy - about 5 - 10 minutes. Add half of the icing sugar and, starting on low speed and working up to high, beat until light and smooth.
Add the remaining icing sugar, and beat, starting on low, until smooth and combined. Add the tequila, lime juice, and salt, and beat on high until light and fluffy.
Transfer the frosting to a piping bag, and frost the cooled cupcakes. Garnish with lime slices and zest.
Notes
*Make your own cake flour! Measure out 1 and 1/2 cups (190g) of plain/all-purpose flour, remove 3 tablespoons, and replace with 3 tablespoons of cornflour/cornstarch. Sift well.Leftover cupcakes can be stored, in an airtight container, at room temperature or in the fridge for up to 3 days. Frosted or unfrosted cupcakes can be frozen for up to 3 months. Thaw overnight in the fridge.Cupcakes can be made 1 day in advance, covered, and stored at room temperature. Cover the frosting, and store in the fridge until ready to use.