Follow these simple, step-by-step instructions to create your delicious microwave snickerdoodle mug cookie in just a few minutes. The key is to mix the wet and dry ingredients separately before combining them to ensure a smooth, lump-free dough.

1. Prepare the Cinnamon-Sugar Topping: In a small bowl or on a small plate, combine the 1 teaspoon of granulated sugar and ¼ teaspoon of ground cinnamon. Mix them together thoroughly with a fork or your fingers. Set this mixture aside for later. This topping will create the iconic crunchy and flavorful crust on your cookie.
2. Choose Your Mug: Select a standard-sized, microwave-safe coffee mug (approximately 8-12 ounces). A wider, shallower mug often works best as it allows the heat to distribute more evenly, but any standard mug will do. Ensure there are no metallic paints or rims on the mug.
3. Mix the Wet Ingredients: In your chosen mug, add the 1 tablespoon of melted unsalted butter, 1 tablespoon of milk, and ¼ teaspoon of vanilla extract. Whisk these ingredients together vigorously with a small fork or a mini whisk until they are well combined and emulsified. This should take about 20-30 seconds.
4. Combine the Dry Ingredients: In a separate small bowl, add the 4 tablespoons of all-purpose flour, 2 tablespoons of granulated sugar, ¼ teaspoon of cream of tartar, ⅛ teaspoon of baking soda, and the pinch of salt. Whisk these dry ingredients together thoroughly. This step is crucial because it ensures that the leavening agents (baking soda and cream of tartar) are evenly distributed throughout the flour, which will result in an evenly cooked and risen cookie.
5. Create the Dough: Pour the combined dry ingredients from the small bowl directly into the mug with the wet ingredients. Using your fork, gently stir everything together until just combined. Be careful not to overmix. The mixture will look like a thick, soft cookie dough. Stop mixing as soon as you no longer see dry streaks of flour. Overmixing can develop the gluten in the flour, leading to a tough, rubbery cookie.
6. Form and Top the Cookie: Gently pat down the dough in the bottom of the mug with the back of your spoon or your fingertips to create a relatively flat, even surface. Now, take the cinnamon-sugar mixture you prepared earlier and sprinkle it evenly over the entire surface of the cookie dough. Don’t be shy with it; this layer is key for flavor and texture.
7. Microwave to Perfection: Place the mug in the center of your microwave. Cook on high power for 60 to 90 seconds. Microwave wattages vary significantly, so it’s best to start with a shorter time. A lower-wattage microwave (around 700-900W) may need the full 90 seconds or even a bit more, while a higher-wattage microwave (1000-1200W) will likely be done closer to the 60-second mark.
8. Check for Doneness: The cookie is done when the edges look set and dry, but the very center still appears slightly soft and under-baked. This residual heat will continue to cook the center as it rests, ensuring a perfectly chewy and gooey middle. If it still looks very wet after 90 seconds, microwave in additional 10-second intervals until it reaches this stage.
9. Let It Rest: Carefully remove the mug from the microwave—it will be very hot! Let the mug cookie rest on the counter for at least 2-3 minutes. This is a vital step. It allows the cookie to finish cooking and set up properly, and it prevents you from burning your mouth on the molten-hot center.
10. Enjoy! Your delicious, single-serving snickerdoodle cookie is now ready to be enjoyed. Grab a spoon and dig in straight from the mug for the ultimate warm and comforting dessert experience.