Combine the cream, sugar, salt, corn syrup or agave, and cocoa powder in a medium saucepan. Bring to a gentle boil over medium heat. Reduce heat to low and simmer for 5 minutes, stirring frequently. Remove from heat and stir in the butter, espresso powder, and vanilla. Let cool. The sauce will thicken as it cools. Stir again before serving. Pour the sauce into a glass jar with a lid. Let cool completely and store, covered, in the refrigerator for up to 2 weeks.

To reheat the sauce for serving, remove the lid from the jar and microwave for a few seconds or remove the lid from the jar and place the jar in a pan of simmering water until warm.

To make this recipe dairy-free and vegan: Use dairy-free milk of your choice (or even water for a lower fat content) and omit the butter. The sauce will be thinner but still fabulous.