In a large mixing bowl, combine the dry ingredients - all-purpose flour, ground cinnamon, baking powder, salt, and sugar.
In a pyrex jar, add the wet ingredients - milk, egg, vanilla extract, and melted butter. Make sure the butter is slightly cool before adding it to the egg-milk mixture, else the egg will curdle. Mix well.
Grate a Granny Smith Apple. Alternatively, you may use a Honeycrisp Apple too. Make sure your apple is tart, and not too sweet. Set aside.
Pour the wet ingredients into the dry ingredients. Mix the batter well, but DO NOT OVERMIX.
Add the grated apple. Fold the apple into the batter.
Heat up a 12" castiron skillet on low-medium heat. Let it heat up for about 3 minutes until it's nice and hot.
Using a 1/4 cup measurement, pour the batter onto the castiron skillet. Cook for 1-2 minutes. Check to see if the other side is slightly golden-brown.
Once bubbles form on top, get ready to flip.
Perfect. Cook on the other side for 1-2 minutes max on low-medium heat.