In a microwave safe bowl, melt the butter. Then add the brown sugar and heavy cream and whisk together until sugar is dissolved. Microwave for 2 minutes. Remove from microwave and whisk more. Microwave once more for 2-3 minutes. The mixture will be bubbly and will have thickened slightly. The sauce will seem runny at first but it thickens as it cools.
**If you want a salted caramel flavor, add a pinch of kosher salt.**