In a medium saucepan, melt butter over medium heat. Add sugars and 1/2 cup heavy cream. Whisk until mixture reaches a boil, then simmer for 3 minutes until temperature reaches 220°F. Add remaining heavy cream and whisk until blended. Remove from heat.

Pour the caramel sauce into a jar and let it cool at room temperature. Refrigerate for up to two weeks. Reheat as needed before use.