Preheat oven to 350F. Line a muffin pan with muffin liners. Set aside.
For the cinnamon streusel topping
In a bowl, combine the flour, rolled oats, sugars, cinnamon, baking powder and salt.
Melt the butter and pour over the flour-oat mixture. Stir to combine. Set aside.
For the apple sour cream coffee cake muffins
In a small bowl, combine the dry ingredients: flour, baking powder, baking soda, salt and cinnamon. Set aside.
Peel and grate the apple. Set aside.
In an electric stand mixer fitted with the paddle attachment, cream the softened butter and sugar together on medium speed until light and fluffy, about 5 minutes. Scrape down the paddle and sides of the bowl.
On low speed, add the eggs one at a time until each one is incorporated. Increase speed to medium until light and fluffy. Scrape down the paddle and sides of the bowl.
Next, we will follow the muffin method alternating between: dry, wet, dry, wet, dry. On low speed, add ⅓ of the flour mixture until about 85% incorporated. Add ½ of the sour cream. Scrape down the bowl. Add another ⅓ of the flour mixture until 85% incorporated. Add last ½ of the sour cream. Scrape down the bowl. Add the final ⅓ addition of the flour mixture until 85% incorporated. Add the shredded apple and finish mixing by hand with a rubber spatula until fully combined.
Scoop the muffin batter into the prepared muffin cups, filling to the top. Pack the streusel topping on top of the muffin batter and gently press down so it sticks while baking.
Bake at 350F for 20-22 minutes, or until a cake tester comes out clean. Let cool in muffin pan on top of a wire cooling rack.
For the glaze
While the muffins are cooling, make the glaze. In a bowl, combine the confectioners’ sugar, milk, vanilla and a dash of ground cinnamon. Stir to combine. Drizzle over the top of the muffins.