Preheat the oven to 175 C (160 C fan). Line 12-hole cupcake tin with paper liners and set aside.
Place butter and sugar in a large bowl, and using an electric mixer, beat together until pale and fluffy, about 3-4 minutes. Add lime zest and mix again to combine. Add eggs, one at a time, along with 1 tablespoon of flour between each addition (it will prevent the batter from curdling), and mix on low speed until well combined. Fold in the remaining flour, followed by gin and tonic and mix together to combine, until you have smooth batter.
Divide the cupcake batter between the cases (fill ¾ of each cupcake case with batter, leaving enough room for them to rise) and bake for 17- 20 minutes until the cupcakes have risen and are springy to the touch. Let them cool for 5 minutes, then remove from the tin and place on the cooling rack.
While the cupcakes are baking, prepare the syrup. Place all the ingredients in a small saucepan and bring to boil. Simmer the mixture for about 10 minutes, until all the sugar dissolved and it becomes a bit thicker. Remove from the heat and set aside.
Brush the cupcakes with the syrup (you can also dip the top of each cupcake in the syrup, or spoon it on top of each cupcake) and set aside, while you prepare the butter icing.
To make butter icing, place the butter and icing sugar in a bowl of an electric mixer and beat on low speed until well combined. Add 2 tablespoon of gin, and mix again until you have smooth, thick and creamy icing. The longer you beat it, the smoother and lighter in appearance it will be. Transfer the icing to a piping bag fitted with the nozzle (I used Wilton 1 D) and pipe it on top of each cupcake. Decorate the cupcakes with some lime zest, lime slices and paper straws.
If you don't have any self-rising flour, you can use plain/ all-purpose flour. Use the same amount as the recipe calls for, but add 2 teaspoon of baking powder to it.
I used 8 tablespoon of gin in total in this recipe, but you can adjust this amount to your tasting.
If you have no limes, you can also use lemons in this recipe. Replace the lime zest with lemon zest, and lime juice with lemon juice.
Use lower shelf of the oven, instead of the middle one, if your oven has a tendency to over-bake.