Preheat oven to 375°F. Grease a 12-cup muffin tin or fill with muffin liners. Set aside.
In a mixing bowl, whisk together the sweet potato casserole, sugar, oil, eggs and vanilla. Mix until smooth, breaking up any clumps of sweet potato.
In a separate mixing bowl, whisk together the remaining muffin ingredients.
Add dry ingredients to the wet ingredients and stir until just combined and moistened. The batter will be lumpy. Do not over mix or muffins will be tough.
Distribute batter evenly amongst muffin tins. Top with streusel if using.
To make streusel, combine all ingredients together. Using clean hands, use tips of fingers to rub the butter into the other ingredients, creating a coarse meal. Alternatively, add all the streusel ingredients into the bowl of a food processor and pulse to form a coarse meal. Sprinkle about 1 tablespoon onto each muffin.
Bake for 20-25 minutes or until a toothpick inserted into the center of the muffins comes out clean. Allow muffins to cool in pan for 1 minute, then remove to a cooling rack.