In a large coffee mug, stir together the 1/3 cup flour, 1/3 cup sugar, 3 tablespoons unsweetened cocoa powder and pinch of salt.
Then stir in the 1/4 cup milk, 3 tablespoons unsalted butter and 1/4 teaspoon vanilla extract. Make sure the butter is in small cubes so it melts evenly.
Stir until all dry mixture is slightly damp. There will clumps of butter.
In the microwave, heat for 20 seconds on high. Remove and stir well. The butter should be melted.
Heat for another 1 minute 15 seconds on high heat. Check and if center is still wet, heat for 15 second intervals until it is fully cooked.
Finish with your favorite toppings.
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