In a tall, microwave-safe mug, melt the butter and chocolate together, microwaving for 15 seconds at a time, stirring after each heating, until completely melted.

Add sweetener, baking powder, almond flour, cocoa powder, and salt. Stir to combine.

Add the egg and vanilla extract and mix until completely combined.

Microwave for 45- 50 seconds. Be sure not to overcook - the cake is still slightly uncooked so the center and top will be "molten'.