In a very large bowl, whisk together all dry mix ingredients (flours, sugar, baking powder, baking soda, salt, and nutmeg, if using), stirring well to ensure ingredients are well-incorporated. Keep in an airtight container for up to 3 months (I'm sure it would last longer, we've just always used it up before then).
To make the pancakes: In a medium bowl, whisk the buttermilk/milk, eggs, melted butter, and vanilla, if using. Add pancake mix and stir until well incorporated. Do not over-mix. Heat a non-stick griddle or large frying pan over medium heat. Using a ⅓ cup measure, pour the batter onto the griddle or into the frying pan. They’re ready to flip when they start to look a little dry around the edges, and the pancakes start to form little bubbles, about 1 minute. Flip and cook on the other side until golden brown and the pancake is cooked through.