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Heat the oven to 180C/160C fan/gas mark 4. Greese and line 2 x 20cm cake tins. In a freestanding electric mixer, or using an electric whisk, beat together the butter and 200g of the sugar until pale and fluffy, for around 5 mins. Add the eggs one by one, making sure they are fully incorporated before adding the next one. If the mixture looks like it might split, add a tablespoon of your flour, then fold in the rest of the flour. Mix the natural yogurt with the juice of one of the limes and 50ml of the gin, then add this to the cake mixture to make a thick and silky mixture. Split the mixture between the cake tins and bake for 35 mins until a skewer comes out clean.
Meanwhile, make the syrup. Put the remaining sugar, tonic water, juniper berries and juice of one lime into a saucepan over medium heat. Once the sugar has dissolved, bring to the boil and cook for 5-7 mins, until it’s a thick syrup. Cool for 5 mins, strain then pour in the remaining gin and set aside.
Once the cake is out of the oven, allow to cool for 5 mins, then prick all over with a skewer, then liberally spoon the syrup mix over both of the cakes. Allow to cool completely in the tin.
To make the buttercream, beat the butter until soft, then add the icing sugar, a little at a time to avoid a sugar cloud! Once fully incorporated, add the milk and the zest of the limes.
To assemble, place one of the cake layers on a cake board, cover with 1/3 of the buttercream then sit the second cake layer on top. Cover this, too, in just a thin coat of icing then put the cake in the fridge for 30 mins to firm up – this will make it easier to get the rest of the icing on smoothly. Use the remaining buttercream to cover the top and sides of the cake.
To decorate, mix the lime zest with the sugar. Sprinkle this over the cake and finish with lemon sherbets, lime slices, cucumber ribbons, party straws and any other gin & tonic paraphernalia you fancy.