Brownie in a Mug

Place the butter and chocolate chips in a mug (an 8-oz/120-ml) mug is the perfect size, but larger will work too) and melt in 20-second increments in the microwave, on high power, stirring in between, until the chocolate is mostly melted, about 40 seconds total. Using a small spoon, stir until completely smooth. Stir in the sugar, vanilla, and salt. Add the egg yolk and stir until smooth. Stir in the flour until smooth and shiny. Using a paper towel, wipe down the batter on the inside edges of the mug.

Microwave, uncovered, on high power for 45 to 55 seconds until puffed and set. (You may need to adjust the time for less or more powerful microwaves.)

Let the brownie rest for 5 minutes. Serve in the mug with a scoop of vanilla ice cream or a dollop of whipped cream.