Stir all-purpose flour, coconut flour, cocoa powder and baking powder together in a small bowl; add 3 tablespoons almond milk, vanilla extract, Nutella(R) and 1/2 tablespoon milk, respectively, stirring between each addition until you finally have a smooth batter. Transfer batter to mug.
Cook in microwave for 1 minute. Allow to rest for about 5 seconds and continue cooking until a toothpick inserted in the center comes out clean, about 30 seconds more.
Let cake cool for 2 to 4 minutes.
You can substitute 7/8 tablespoon whole wheat flour for the all-purpose flour.