In a microwave-safe mug (12-14 ounces), add the melted butter, peanut butter, brown sugar, milk, and vanilla extract.
Whisk vigorously until the mixture is well blended.
Add the flour, baking powder, cinnamon, and salt.
Mix until smooth, making sure no flour lumps remain.
Stir in the chocolate chips, if using.
Microwave on high for 60-70 seconds, until the top looks set.
Let the cake rest for a minute or two, then enjoy warm.