For topping: Combine brown sugar, flour, and cinnamon in a small bowl. Gently stir in melted butter just until flour is moistened and mixture is crumbly. Refrigerate while you prepare the muffin batter.
Spray 12 muffin cups with non-stick cooking spray. Preheat oven to 425°. Peel and chop the apples.
In a mixing bowl, whisk together the flour, baking powder, salt, and cinnamon; set aside.
In a large bowl, whisk together oil, sugar, eggs, buttermilk, and vanilla.
Add the dry ingredients and stir just until barely combined. Gently fold in the diced apples.
Divide batter between the 12 cups. (They will be about 3/4 full.) Sprinkle the cold topping evenly over the muffins.
Bake at 425 degrees for 5 minutes. Turn the heat down to 350° and bake for another 12-15 minutes, or until the middles are set.
Let cool in the pan for about 5 minutes, then remove to cooling racks.
For glaze, whisk all ingredients in a small bowl until smooth, adding cream as needed. Drizzle over the tops of the muffins.