Mix all ingredients in a small bowl and set aside.
To Make the Muffins
Preheat the oven to 420°F. Line a 12 cup muffin pan (OR a 6 cup jumbo muffin pan).
In a large bowl mix dry ingredients well. In a separate bowl whisk together the oil, the eggs and the vanilla extract, then stir in the grated apple.
Add the wet ingredients to the dry ingredients and mix JUST until combined. Do not overmix!
To Assemble
Evenly distribute ¾ of the batter between the muffin cups (I like using a cookie scoop for mess-free work). Roll the filling into balls and gently push one into the center of each muffin. Top with the remaining batter.
Reduce the oven temperature to 400°F. Bake the muffins for 10 minutes for regular muffins (or 15 for jumbo sized ones). Reduce the heat to 360°F and finish baking for 15 minutes for regular muffins (or 20 minutes for jumbo sized ones), or until a toothpick inserted comes out clean.
Cool the muffins in the pan for 10 minutes before removing them to a cooling rack to cool completely.