Break chocolate into chunks. Microwave for thirty seconds with butter. Microwave for an additional 15 seconds if needed. Mix to melt completely.
Combine the egg and sugar in a separate bowl, whisking lightly.
Mix in the egg mixture to the chocolate. Slowly add in the flour. The batter should still be liquid-y.
Optional step: freeze a truffle ahead of time to add into batter. Put the truffle in when the mug is half-filled with batter, then add in the rest of the batter. This creates a molten lava cake.
Spray cooking oil into a mug. Pour in the batter. Microwave for 1 minutes and 30 seconds, then let stand in the microwave for about a minute. (based on a 1100 W)
-you can use different kinds of chocolate bars. I used a sea salt-almond chocolate bar