Grease the insides of two microwave-safe mugs with a little melted coconut oil. In a medium-size bowl, mix the flour, baking powder, baking soda, salt, nutmeg, cinnamon, banana, milk, oil, and vanilla. Add the walnuts and blueberries and fold together. Pour the cake batter into the mugs. Top with an extra sprinkle of chopped walnuts or blueberries, if desired.