Preheat oven to 350°F and line a 12-count muffin pan with cupcake liners or spray with nonstick spray. This recipe makes about 15 standard sized muffins, so you'll actually need two muffin pans or you can bake in batches
To make the muffins: Whisk the flour, baking soda, baking powder, and salt together in a medium bowl. Set aside.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on high speed until smooth and creamy, about 2 minutes. Add the eggs and vanilla extract and beat until the mixture is combined and uniform in texture.
Pour the dry ingredients into the wet ingredients and mix on low briefly. Then, add the milk and mix until everything is combined. Scrape down the sides and bottom of the bowl as needed, and fold in the chopped apple.
To make the cinnamon swirl: Add brown sugar and cinnamon to a small bowl and mix until combined.
Add the swirl mixture to the batter and gently fold a few times, but do not completely incorporate.
Spoon the batter evenly into the prepared muffin tin, filling each cup about 2/3 full.
Bake for 20-25 minutes at 350°F degrees or until a toothpick inserted in the center comes out clean. Allow muffins to cool for 5 minutes before transfering to a cooling rack to cool completely.
To make the glaze: Starting with 1 tablespoon of milk, whisk all of the ingredients together. Continue adding small amounts of milk until you reach your desired consistency and drizzle over cooled muffins.