1. In a heavy six quart saucepan over medium heat, melt the butter.
Use a high-quality pan.
2. Add the evaporated milk, salt, vanilla extract, granulated and brown sugars. Stir with a wooden spoon until sugar is completely dissolved and mixture is smooth.
3. Increase heat slightly, stirring mixture, until the caramel icing comes to a steady rolling boil. Boil for 10-12 minutes, stirring slowly and consistently. While stirring, make sure your spoon stirs the bottom and sides of pan as well. Note that once the icing is boiling, it will nearly double in volume. Watch the caramel icing carefully to ensure it doesn’t boil over. Reduce the heat slightly if mixture is boiling too vigorously.
Patience is key—don’t rush the process!
4. Once the caramel icing is the consistency of warm peanut butter and forms small ribbons, remove it from the heat. You may think the caramel icing is not thick enough yet, but it will continue to thicken quickly as it is beaten and cools.
5. Add vinegar or lemon juice. The icing will foam. Beat with an electric mixer on medium speed for several minutes until caramel icing begins to cool and thickens slightly. Once it begins to thicken, work quickly to fill and frost your cake. If icing becomes too thick, thin with milk.
6. Once the caramel icing is the consistency of warm peanut butter and forms small ribbons, remove it from the heat. You may think the caramel icing is not thick enough yet, but it will continue to thicken quickly as it is beaten and cools.
7. Add vinegar or lemon juice. The icing will foam. Beat with an electric mixer on medium speed for several minutes until caramel icing begins to cool and thickens slightly. Keep in mind that the icing may seem too thin to frost the cake, but it will spread well and then harden on the cake. It’s SOOOO good!
