A standard size mug is fine. I have made this in a 12oz mug and a 16oz mug.
Don’t over mix your ingredients, which can lead to a tough brownie.
Don’t overcook it. Microwaves vary, so start by cooking it 1 minute, and then an additional 15-30 seconds as necessary.
Do not skip the chocolate chips. They make our brownie fudgy.
I will reply with how to make it