1. In a tall airtight container, add the heavy cream and stir in the ground coffee until it’s completely mixed. Close the container with a lid and refrigerate overnight.
2. Strain the coffee mixture, making sure to push through as much milk fat as possible, while keeping out the sediment. Discard the ground coffee and transfer the strained liquid to the base of a food processor. Add the sugar and kosher salt, and spin the mixture until the fat forms into butter and the liquid separates. Transfer the mixture to a large piece of cheesecloth and wring out any excess liquid. Transfer the butter to a small condiment bowl, garnish with flaked salt and and use immediately or store in the refrigerator for later.