Choose the right mug. Use a large, microwave-safe mug (at least 12 ounces) to prevent overflowing and allow the cake to rise evenly.
Mix the dry ingredients. In the mug, whisk 3 tablespoons whole wheat flour, 1/4 teaspoon baking powder, 1/2 teaspoon cinnamon, and a small pinch of salt until well combined.
Stir in the wet ingredients. Add 2 tablespoons unsweetened applesauce, 1 tablespoon maple syrup, 2 tablespoons milk, 1/4 teaspoon vanilla, and 1 teaspoon oil. Stir until the batter is smooth with no flour pockets.
Fold in the apple. Add 2 to 3 tablespoons finely diced apple.
Stir gently so the fruit is evenly distributed. If using nuts or raisins, add a tablespoon now.
Microwave in short bursts. Cook on high for 60 seconds. Check the center; if it’s still glossy or wet, microwave in 10–15 second bursts until the top is set and springs back.
Total time is usually 70–90 seconds, depending on your microwave.
Let it rest. Allow the mug cake to sit for 1 minute. This helps the crumb set and keeps it tender.
Top and enjoy. Add a spoon of yogurt, a sprinkle of cinnamon, or a few extra apple bits. Serve warm right from the mug.