Preheat the oven to 400°F
Spray a muffin pan lightly with non stick spray and then place muffin liners in each space.
In a stand mixer or hand held mixer cream the butter and sugar until light.
Add the eggs one at a time and vanilla, mixing until the mixture has come together, scrape down the sides of the bowl.
Add the sour cream on low speed.
In a separate bowl whisk together the flour, baking powder and cinnamon.
Using a spatula gently fold the flour into the batter and then add the diced apples making sure to fold them through as well.
Distribute the batter evenly between each muffin paper, about a generous ¼ cup.
Bake the muffins at 400° for 5 minutes and then lower the temperature for the duration of baking time to 375° for about 17-20 minutes in total.
Let the muffins cool while you make the icing.
Combine the powdered sugar and milk and mix until smooth and lump free.
Drizzle the icing over the muffins and serve warm.