Preheat oven to 400° and prepare a cupcake pan. Either line with paper cups or spray.
In a large bowl, combine the flour, oats, brown sugar, baking powder, cinnamon and salt. In a separate bowl, combine the milk, melted butter, maple syrup, eggs, vanilla and greek yogurt. Add the wet ingredients to the dry and mix until just combined. Fold in the chopped pecans.
In a separate small bowl, make the crumble topping. Use your hands to combine the small pieces of butter, flour, brown sugar and oats. Break apart into small pieces.
Fill the muffin tins 3/4 of the way full with the muffin batter. Then top with the crumble topping. Bake for 15 minutes or until toothpick comes out clean.
Remove the muffins from the pan when they come out of the oven and transfer to a wire cooling rack. Let cool on rack for 10 minutes.
Combine the icing ingredients in small bowl and drizzle over the muffins using a spoon.