Cut butter into 8 slices and add to a heavy-bottomed medium pot over medium heat. Add sugar, cocoa powder, salt, and cream. Whisk until the butter melts and the mixture is smooth.
Remove from heat, whisk in vanilla, and taste. Add more salt if needed.
Let cool slightly, then blend (in batches if needed) in a high-powered blender for 20–30 seconds until smooth. Pour into glass jars and refrigerate—the sauce will thicken as it cools.
Video
Notes
Note 1:Makes 2 cups but can be halved if you don’t need a full batch.Note 2: The cocoa powder makes all the difference—use Dutch process for the richest flavor.Storage: Keep in an airtight container in the fridge for up to 2 weeks—it thickens as it cools. Reheat in the microwave or on low heat, stirring frequently. To freeze, store airtight, thaw in the fridge, and reheat.