3. In a mixing bowl, combine the sugar and butter and beat on medium-high until light and creamy, about 3 minutes. Add the egg and beating for an additional minute. In a large mixing cup or small bowl, combine the buttermilk, Coke and vanilla extract. (Make sure you wait for the foam to go down when measuring the Coke and don't worry if your mixture looks curdled when you add the buttermilk.)