How to Make It
Preheat oven to 350°F. Butter and flour 3 8-inch round cake pans; set aside.
Make the Cake Layers: In a medium bowl, combine flour, gelatin, baking powder, salt, and baking soda. Beat butter with a heavy-duty electric stand mixer on medium speed until creamy. Gradually add granulated sugar, beating until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating on low speed until blended after each addition. Stir in vanilla extract. Divide batter evenly between prepared cake pans. Bake until a toothpick inserted into the center of each cake comes out clean, 27 to 30 minutes. Allow to cool completely before removing from pans.
Make the Syrup: Combine ingredients in a medium saucepan and bring to a boil. Remove from heat and allow to cool to room temperature.
Make Frosting: Cream together butter, sugar, salt, and lime zest until well combined. Add powdered sugar and combine on low until incorporated. Slowly add margarita mixer and continue to beat on low until combined. Increase speed to medium-high and beat until Frosting becomes airy, about 3 minutes.
Assemble Cake: Brush each cake layer heavily with Syrup. Take first layer and spread 1/3 cup of Frosting evenly across the surface. Add the second layer and spread another 1/3 cup of Frosting atop that layer. Add final layer and cover entire cake with 1 cup of Frosting. Add green food coloring as desired to remaining Frosting and stir to combine. Apply colored Frosting to cake, adding more to some areas and less to others, giving the cake a gradient of color between white and green. Garnish by sprinkling the edge of the cake with flake salt and decorating with candied lime peels.