In a microwave-safe mug, melt the unsalted butter for about 20-30 seconds until liquefied. Let it cool slightly.

Add granulated sugar and cinnamon to the melted butter. Stir until well blended and creamy.

Incorporate vanilla extract and egg yolk into the mixture, mixing until smooth.

Fold in all-purpose flour, baking soda, and salt until just combined.

Add optional chocolate chips or chopped nuts, mixing gently.

Microwave the mug on high for about 1 minute. Check for doneness.

Carefully remove from the microwave and let cool for a minute before enjoying.