1. Preheat oven to 350 degrees Fahrenheit. Line 10–12 cups of a muffin tin with liners and set aside.
2. Prepare the crumb topping. In a bowl, combine all ingredients, using your hands to mix until mixture has a sandy consistency. Set aside.
3. Prepare the muffins. Combine potato starch, tapioca flour, almond flour, sugar, baking powder, nutmeg, cinnamon, and salt) and mix well.
4. In a large bowl, whisk the milk, vanilla extract, and egg. Add dry ingredients and sweet potato mash. Add melted nut butter and mix very well to form a batter.
5. Pour batter into the muffin liners until halfway full. Top with some of the crumbs, then add a little more batter until muffin liner is three-fourths full. Top again with crumbs.
6. Bake for 25–30 minutes or until a toothpick or skewer comes out clean. Remove from oven and let sit for five minutes before removing from pan.