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In a microwave-safe mug, whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.
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Add the milk, vegetable oil, and vanilla extract to the mug, and stir until the batter is smooth and no lumps remain.
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Microwave the mug on high for 1 minute and 30 seconds. (Cooking time may vary depending on the wattage of your microwave, so start with 1 minute and adjust as needed.)
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Carefully remove the mug from the microwave (it will be hot!), and let it cool for a few minutes before serving.
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Optional: top with whipped cream, sprinkles, or other toppings of your choice.