In a tall medium saucepan over medium-high heat, add sugars. Stir until the sugars start to melt. When almost melted, reduce heat to medium and continue stirring until all melted. Let the mixture sits for 15 to 30 seconds; turn the heat off.
Add butter and stir continuously until melted. Add heavy cream and quickly stir until all combined. Add generous pinches of fleur de sel and give another stir to mix.
Remove from the heat and transfer to a sealed jar. Let it cool completely before transferring to the refrigerator.