Pre-heat oven to 400°F. Line a standard sized muffin pan with muffin papers. Set aside.
In a large bowl, combine flour, oats, salt, baking powder and cinnamon.
In a separate bowl, mix together brown sugar and eggs. Add the buttermilk, vegetable oil, Greek yogurt and maple extract. Mix the wet ingredients into the dry ingredients until just combined. Fold in the walnuts.
Prepare the crumbly topping: combine all the ingredients and cut the butter in using a pastry tool until coarse and crumbly.
Fill the muffin papers until ¾ of the way full. Sprinkle a generous spoonful of the crumble topping over top.
Bake for 15-20 minutes, until a toothpick inserted into the center comes out cleanly (mine were perfect at 17 minutes).
Allow to cool 5 minutes in the pan, then transfer to a wire rack to cool completely.
Combine all ingredients using a hand held beater. Add additional milk until you get the desired consistency.
Drizzle the glaze over the muffins using a plastic bag or a spoon.
To avoid dry muffins, make sure to measure your flour correctly: fluff flour, then spoon into measuring cup, then level (or just weigh it!)
Also, make sure not to over-bake the muffins, they can go quickly from still runny to set in no time!