Set oven to 350˚F. Grease and line 8" round cake tin. Set aside.
In a separate bowl sift together flour, baking powder, baking soda, and salt. With mixer on low speed, gradually incorporate dry ingredients into wet mixture until just combine.
In the bowl of a stand mixer with the whisk attachment combine sugar, oil, eggs, lime zest, and vanilla. Whisk until ingredients are evenly distributed.
Add gin, lime juice, and ground juniper berries to cake batter and whisk in until smooth. Remove bowl from stand mixer. Add tonic water and gently fold into batter.
Pour batter into prepared cake and bake until a tester comes out clean, roughly 40-45 minutes. Cool cake slightly and remove from tin. Cool completely.
In a saucepan over low heat combine powdered sugar, gin, and lime juice. Whisk together and cook until warmed through. Remove from heat. Pour half of glaze over cooled cake and let set to absorb roughly 1 minute. Pour remaining glaze over cake. Cut and serve as desired.
This cake batter is perfect to make cupcakes for individual service.