1. Microwave butter and chocolate in a small bowl at 50% power until just melted, about 1 minute, stirring halfway through.
2. Whisk in sugar, egg, cocoa powder, vanilla, and salt until smooth, then stir in flour and baking soda until blended. Scrape batter into a 12-ounce microwave-safe mug.
3. Microwave at 50% power for 45 seconds, then stir well. Microwave at 50% power for 1 more minute (cake should be firm but still slightly fudgy on top); let sit 2 minutes. Top with whipped cream, mini marshmallows, and/or chocolate syrup.
Source: Hannaford fresh Magazine, October - November 2021