Preheat your oven to 350°F and line a standard muffin tin with liners. In a small bowl, combine cinnamon and granulated sugar.
To make the brown sugar swirl, stir together the brown sugar, flour, sea salt, and cinnamon.
Peel, core, and finely chop the apple. You want the pieces to be a nice dice as you would an onion. Set both aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the room temperature eggs along with the cooled melted butter, milk, and vanilla at medium-low speed until well combined.
In a large mixing bowl, whisk together flour, both brown and granulated sugars, baking powder, ground cinnamon, and sea salt.