Make the crumb topping: In a medium bowl, combine both sugars, the cinnamon, and melted butter. Using a rubber spatula, stir in the flour. The crumb topping will be thick and crumbly. Set aside.
Preheat oven to 425°F. Spray a 12-count muffin pan with nonstick spray or line with cupcake liners.
Make the muffins: Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and both sugars together on high speed until smooth and creamy, about 2 minutes. Scrape down the sides and bottom of the bowl as needed. Add the eggs, yogurt, and vanilla extract. Beat on medium speed for 1 minute, then turn up to high speed until the mixture is combine. Scrape down the sides and bottom of the bowl as needed.
Whisk the flour, baking soda, baking powder and salt together in a medium bowl. Pour the dry ingredients into the wet ingredients, add the Apple Cinnamon Protein Shake with Oats. and mix on low speed until everything is combined.
Spoon the batter evenly into each cup or liner, filling each all the way to the top. Press a handful of the crumb topping into the top of each; crumble it with your hands to make some big chunks.
Bake for 5 minutes at 425°F, then keeping the muffins in the oven, lower the oven temperature to 350°F and bake for 12-15 more minutes or until a toothpick inserted in the center comes out clean. Make the icing: whisk all of the ingredients together and drizzle over warm muffins.
Muffins stay fresh stored at room temperature for a few days– then transfer to the fridge for up to 1 week.