Preheat oven to 375 degrees Fahrenheit.
Butter a 9-inch square pan and set aside.
Combine flour, oats, brown sugar, butter and cinammon if you're using it in a large bowl.
Use a spoon to press about half of the crumb mixture into the bottom of the pan.
Add baking powder and salt to the remaining crumb mixture and mix it well.
Make a well in the middle for your wet ingredients.
Whip together the egg, sour cream, and vanilla extract.
Pour wet ingredients into the well and mix gently.
Fold in other chopped apples or other fruit.
Use a spatula to scrape the coffee cake mixture into the pan on top of the crumb base.
Bake for 60 minutes then insert a knife into the centre. If it comes out clean, it's ready. If not, pop it back in the oven for another 5 minutes then repeat.