Preheat oven to 400 degrees. Pierce sweet potatoes with a fork, then place on a baking sheet. Roast for 45-60 minutes, until fork tender.
Allow sweet potatoes to cool, then remove skins. Place in a bowl and mash with a fork (should be 1 ¾ - 2 cups). Reduce oven temperature to 350 degrees.
Line a 12-cup muffin tin with muffin liners and coat with cooking spray. Set aside. In a medium bowl, use a hand mixture to whisk together mashed sweet potato, stevia/sugar blend, eggs and vanilla extract, until incorporated.
Whisk in evaporated milk until combined. In a separate medium bowl, whisk together flour, pumpkin pie spice, salt, baking powder and baking soda until combined.
Whisk wet ingredients into flour mixture until smooth. Pour batter into each muffin cup about ¾ of the way full. In a small bowl, stir together graham cracker crumbs, melted butter and stevia/sugar blend.
Distribute graham cracker crumble on top of each cup of muffin batter. Bake 18-22 minutes, until cakes are set. Allow to cool before eating.