Select two mugs that hold at least 11oz of liquid in them (about a cup and a half).
Microwave the butter and chopped bittersweet chocolate in a large bowl for about 1 minute, stirring halfway through, until fully melted. Set aside.
Whisk the sugar, eggs, cocoa powder, vanilla, and salt in a medium bowl, then whisk this into the melted butter and chocolate. Whisk in the flour and baking powder, then evenly divide the batter between the two mugs.
Put the mugs on opposite sides of the microwave turntable, and microwave at 50% power for 45 seconds. Stir the batter well with a spoon, then microwave for another 45 seconds at 50% power.
If you're adding extra chocolate to the tops of the cake (see recipe notes), now is the time to press one square into the top of each cake. As stated in the blog post, I personally don't add extra chocolate.
Microwave for an additional 35 seconds at 50% power. At this point the cake should still be a little gooey on top, but you can check that the interior of the cake should be around 200 degrees F. Let the cakes sit for 2 minutes while the heat continues to cook the cakes slightly, then serve with vanilla ice cream. Enjoy!
This recipe was developed for a full-size, 1200-watt microwave, according to Cook’s Illustrated (as stated above, this recipe is adapted from their issue published in 2013). This is the vanilla paste I use now instead of vanilla extract. I love the little vanilla bean specks!*If you wish, you may add two 1/2 oz pieces of bittersweet chocolate to press into the tops of the mug cake.If only cooking one mug cake, you may need to reduce the cook time slightly. Try reducing to three 30-second intervals, keeping your eye on the cake. Be careful not to overcook.