Preheat the oven to 180/160 mark 4. Grease a 2lb loaf tin and line it with baking parchment. Put your butter and caster sugar in a large bowl and beat together until nice and fluffy (probably take 5 mins with an electric hand mixer) Add the eggs, beating again each time you add one.
Fold in the flour, baking powder and lemon zest. Spoon into the tin and bake in the oven for 55 minutes, or until a skewer comes out clean. While the cake is cooking, make the sugar syrup.
Pour the tonic in a small saucepan and add the caster sugar – heat it gently until it dissolves, then boil it for a minute. Take 2 tablespoons out and add 2 tablespoons of the gin in another container – this is to soak the cake with once it’s cooked.
Use the syrup to candy the lemon – cut it into slices and simmer them for 10+ minutes until they look translucent, then take them out carefully and lay them on sugared baking parchment, and scatter more sugar on top.
Once the cake is cooked, poke it full of holes and pour the syrup evenly over it and let it soak through as it cools (leave it in the tin on a wire rack)
While the cake cools, use the rest of the gin and sifted icing sugar to make the icing – add more gin if it’s too firm, then decorate with your candied lemon slices. G&T heaven – in a cake!