01 - Preheat oven to 375°F. Line a 12-cup muffin pan with paper liners or lightly grease each cup.
02 - In a small bowl, combine the flour, brown sugar, and cinnamon for the streusel. Cut in cold diced butter until coarse crumbs form, then refrigerate.
03 - Mix granulated sugar and ground cinnamon in a separate small bowl and set aside.
04 - In a medium bowl, whisk together all-purpose flour, baking powder, baking soda, salt, and ground cinnamon.
05 - In a large bowl, beat softened unsalted butter and granulated sugar until light and fluffy. Blend in eggs one at a time, then add vanilla extract.
06 - Gradually stir in half of the dry ingredient mixture, then pour in the whole milk. Add the remaining dry ingredients and mix until just combined.
07 - Gently incorporate the chopped apples into the muffin batter using a rubber spatula.
08 - Fill muffin cups halfway with batter. Sprinkle with half the cinnamon sugar mixture. Top with the remaining batter and finish with the remaining cinnamon sugar.
09 - Evenly distribute the chilled streusel topping over each muffin.
10 - Bake for 20 to 22 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
11 - Let muffins rest in the pan for 5 minutes after baking, then transfer to a cooling rack until completely cooled.