Add butter to your mug. Microwave until melted. Stir vanilla extract, milk and sugar into the butter. Add flour, baking powder and salt (and sprinkles if using) and stir with a fork until combined and you have a smooth batter. (Make sure to scrape the sides and the bottoms to get any hidden clumps.)
Microwave mug cake 1 minute to 2 minutes. (I found 1 minute and 10 seconds in the sweet spot for my microwave.) Be careful removing the mug from the microwave as it may be hot.
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Notes
Note: I've used whole and 2% milk when testing the recipe but you can use whatever milk you have on hand, whether it's whole, skim, or almond milk.Use a fork to mix! This helps break up clumps that could cause a gummy texture.