To make the syrup, combine all of the ingredients except the tequila in a saucepot and bring to a boil, stirring constantly until the sugar has melted. Remove syrup from heat, add the tequila, and cool to room temperature.
For the icing, place the sugar, salt, and lime zest in a bowl. Add the liquids a little at a time until the mixture is just loose enough to pour. Be careful not to add too much liquid, but if you do, add more sugar as needed.
To finish, hold the cake upside down and cut a little well around the bottom of the cake. Pour half of the syrup evenly over the cake. Turn the cake over, and using a pastry brush, paint the cake with the remaining syrup.
Drizzle the icing over the cake. Sprinkle with candied lime zest. Slice and serve with whipped cream.
Recipe courtesy of Daniel Orr, Gastavono's