Lightly butter 4 small microwave-safe mugs. Whisk together the buttermilk, sugar, oil, egg and vanilla in a large bowl until smooth, about 2 minutes. Whisk in the flour, cocoa powder, baking soda and salt until just combined.
Pour 1/3 cup batter into each mug. Microwave, one at a time, until puffed and a toothpick inserted into the center comes out clean, about 2 minutes. (Continue microwaving in 15-second intervals, if needed.) Let cool slightly. Top as desired.
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