This is the classic icing that is spread onto yellow layers to make the Delta Caramel Cake. It’s a laborof love to make, and it is delicious. For the faint of heart, opt for Quick Caramel Icing
Place 2½ cups of the sugar, the evaporated milk, and salt in a large heavy saucepan over medium heat.
Stir constantly until the mixture boils and the sugar has dissolved, and continue to stir and let the mixture boil for 3 minutes.
Remove the pan from the heat.
Place the remaining ½ cup sugar in a small cast-iron skillet over medium heat.
Let the sugar caramelize—turn from granulated into a deep golden brown—without stirring, 4 to 5 minutes.
When the mixture is just deep golden, remove it from the heat.
Do not let it become dark brown.
The heat in the skillet continues to cook the sugar, so if you let it get dark, it will burn.
(If this happens, remove the skillet from the heat, let it cool completely, and clean the skillet. Repeat the process with a clean skillet and ½ cup granulated sugar.)
Stir about ½ cup of the sugar and evaporated milk mixture into the hot caramelized sugar to bring the heat down.
Then pour the contents of the iron skillet back into the saucepan with the remaining sugar and evaporated milk mixture.
Stir until incorporated.
Add the butter and vanilla, and stir until the butter melts and the mixture is smooth.
Place the saucepan into a large bowl filled with 2" of ice water.
Stir the icing with a wooden spoon until it cools and thickens, 4 to 5 minutes.
You don’t want the icing to thicken too much or it will be difficult to spread.
When it is of a spreadable consistency, nearly as thick as peanut butter, remove the pan from the ice water bath.