In a microwave-safe mug (at least 12 oz), whisk together the whole wheat flour, cocoa powder, baking powder, and salt until well combined.
Add the maple syrup, almond milk, applesauce, and vanilla extract to the dry ingredients.
Stir the mixture with a small whisk or fork until the batter is smooth and no flour pockets remain.
Place the mug in the center of the microwave and cook on high for 75 to 90 seconds.
Check for doneness: the top should be set but slightly tacky to the touch.
Remove carefully and allow the cake to rest for 2 minutes to finalize the internal structure before serving.