Grease two 8-ounce coffee mugs, ramekins or microwave-safe bowls with butter. In a small bowl, stir together the egg, heavy cream, butter, and vanilla extract until combined. In a medium bowl, whisk together the dry ingredients, including the chocolate chips, if using. stir the wet ingredients into the dry until combined.
Divide the batter evenly between the two prepared microwave-safe mug.
Microwave one brownie at a time in 30-second intervals until the center is no longer wet and a toothpick inserted in the center comes out clean. (Mine was done after 1 minute of cooking on high power.)
Store covered in the refrigerator for up to 1 week. Reheat in the microwave for 30 seconds or enjoy chilled from the fridge.
To make in the oven instead of microwave, see notes above.