Place the brown sugar, butter, and milk in a small pan over medium heat. Stir continually. This part is tricky because you want to melt but not burn the sugar. To make sure you don't burn it, stir continually and when the mixture starts to boil, only let it boil for a minute before removing from heat. If you boil any longer than this, your sauce may end up being too hard to eat after you pour it on your ice cream.

Once the sugar is completely melted and blended with the butter and milk, remove from the heat. Add in the vanilla. Whisk together and pour into a glass jar and store in the fridge.

When you're ready to use it on your ice cream, warm the mixture up to make it a bit softer again.