Instructions:
- Melt the butter in a medium, heavy-bottomed saucepan over medium-low heat until just liquid. Do not allow it to brown.
- Whisk in the granulated sugar, cocoa powder, salt, and espresso powder (if using). Stir continuously for approximately 1 minute to toast the cocoa slightly.
- Gradually whisk in the heavy cream until the mixture is completely smooth and there are no dry lumps of cocoa remaining.
- Bring the mixture to a gentle simmer, stirring frequently. Continue to simmer for 3–5 minutes until the sauce slightly thickens and coats the back of a spoon.
- Remove the saucepan completely from the heat and allow it to stop bubbling vigorously (about 30 seconds).
- Whisk in the pure vanilla extract until fully incorporated. Taste and adjust seasoning if necessary (e.g., a tiny pinch more salt if it tastes flat).
- Pour the finished sauce into a clean, airtight jar. The sauce will thicken to a perfect, spoonable consistency as it cools to room temperature.