- Preheat oven to 375°F. Line 12-cup muffin pan with paper liners.
- To make streusel, stir together brown sugar, oats, pecans, and cinnamon in small bowl until blended. Stir in oil.
- To make muffins, whisk together all-purpose flour, whole wheat flour, granulated sugar, brown sugar, baking powder, ginger, baking soda, cinnamon, and salt in large bowl. Whisk together sweet potato, buttermilk, oil, and egg in medium bowl. Add sweet potato mixture to flour mixture and stir until moistened (batter will be thick).
- Spoon batter into prepared muffin cups and sprinkle evenly with streusel. Bake until toothpick inserted into centers of muffins comes out clean about 20 minutes. Cool in pan on wire rack 10 minutes. Remove muffins from pan and cool completely on rack.
- Per serving: 1 muffin
To get 1 cup of mashed sweet potato, bake 1 large sweet potato at 400°F for 1 hour or until tender. Cut lengthwise in half and open up to let cool before scooping flesh out and mashing.