Add the 2 cups of sugar and butter to a deep- bottomed saucepan initially. Make sure the saucepan is kept at medium heat as there is a chance of the mixture sticking to the bottom of the saucepan at high temperatures.
Whisk the mixture carefully with a wooden spatula. Stirring the mixture with a regular cooking spoon would not properly flatten any lumps produced during cooking.
Continue to stir the filling in the saucepan until the colour turns to golden brown. To achieve the desired colour, constant stirring is required as the mixture has to be dissolved completely. Also, if the spatula is not constantly stirred in the mixture, there is a chance that the icing could stick to the bottom of the saucepan.
When the butter and sugar are completely dissolved and the colour turns golden brown, add the heavy cream to the saucepan. Continue cooking the icing until it comes to a boil.
When the filling reaches a boil, add the brown sugar to the saucepan to make the icing more thick. In case, the mixture has not thickened to the level required some water or baking soda can also be added.
Remove the mixture from the saucepan and pour it in a bowl. The filling has to be put at room temperature for the next 30 minutes so that it cools down. Make sure the icing is not cooled in the refrigerator as that could cause blocks in the mixture.