Put the butter in a small microwave-safe bowl or ramekin, and microwave at 50% power for 30 seconds. If it’s not melted after that, continue to microwave in 10-seconds bursts until it is.
Place the flour, sugar, cocoa, salt (if using unsalted butter), and optional cinnamon in a microwave-safe ceramic mug. Stir with a fork until it resembles brown sand and there are no clumps.
Stir in the butter (or oil) with a fork until a paste forms. Then add the milk (or water or coffee) and vanilla. Stir with the same fork until smooth.
Microwave, uncovered, on high power for 1 minute. Adjust the time for less or more powerful microwaves.
Let the brownie rest for 4 minutes.
Serve with a scoop of vanilla ice cream or a small dab of cream.
