For the cupcakes, preheat oven to 180°C / 160°C fan assisted / gas 4. Line a muffin tin with 12 paper muffin cases. Tip: There is enough batter to make 14 cupcakes, so avoid over-filling your cupcake papers, and instead, bake two extra for the cook to taste.
Beat butter and sugar in a large bowl until well creamed. Add the eggs, one at a time, beating until each well is well incorporated before adding the next one. Add the flour, lime zest and salt, mix in just until combined. Divide batter between cupcake papers, each one about 2/3 full. Bake in pre-heated oven until toothpick inserted into the centre of cupcakes comes out clean, about 18-22 minutes.
While cupcakes are baking, prepare the syrup. Combine tonic water, sugar and lime juice in a medium sized pot. Bring to a boil, then reduce heat slightly and leave on a rolling boil until liquid is reduced by half and it thickens to a thin syrup. This can take between 6-10 minutes. Keep an eye on it as it boils, the closer it gets to becoming a syrup, the higher the bubbles start to rise, so you may need to lower the temperature a bit, or stir occasionally to keep the bubbles from spilling over the sides of the pot. It’s ready when the bubbles are large and glassy and take longer to pop, it should be similar to maple syrup when poured. Remove from heat and leave to cool while the cupcakes bake with the pot sitting on a wire cooling rack or trivet. Just before the cupcakes are about to come out of the oven, add the gin and stir well to combine.
Remove cupcakes from the oven and leave to cool for about 5 minutes. Poke the cupcakes with a skewer and spoon syrup over each cupcake, 1 tsp at a time to give the syrup time to soak into the cupcake. Each cupcake will get about 1 ½ - 2 tsp of syrup.
Leave to cool completely. For the frosting, beat butter in a large bowl with an electric mixer until smooth. Add the icing sugar, vanilla and 3 tbsp of the gin. Start beating together with a mixer on the lowest speed, once most of the sugar has been mixed in, increase speed and beat until frosting is smooth and creamy. Make sure to scrape down the sides of the bowl so that everything is well incorporated.
While beating the buttercream on low speed, add the last 1-2 tbsp of gin (to taste), pouring each tablespoon in slowly while you mix. To decorate your cupcakes, fill a large piping bag fitted with a large plain piping tip with the frosting. Pipe swirls of buttercream onto cooled cupcakes. If you prefer, you can also simply spread the frosting on with a knife. Decorate however you like! We used white sugar sprinkles, and just before serving, topped off each cupcake with a small wedge of lime or a cucumber shaving or a bit of lime zest.