Preheat oven to 350*
Make crumb topping by mixing oats, brown sugar and whole wheat flour in a small bowl. Then, cut in the butter, using two forks or your hands to mix until small crumbly pieces are formed. Set in refrigerator until ready to use.
In a medium bowl whisk together sweet potato puree, yogurt, sugar, eggs, butter, oil, and vanilla until completely combined and smooth.
In a large bowl, whisk together whole wheat flour, chai seasoning and baking powder.
Add the sweet potato mixture slowly into the flour mixture and stir until well combined and a smooth batter forms.
Scoop about 1/3 cup of batter into each well of a greased muffin tin. Then distribute the crumb topping evenly over each muffin. Press down gently to ensue the crumb adheres to the batter
Bake at 350* for about 20 minutes or until a toothpick inserted in the center comes out clean. Let the muffins cool at least 5 minutes in the tin before removing and storing.
For the sweet potato puree: I roasted whole sweet potatoes at 400* until soft. Let cool, remove the skins and puree in a food processor until very smooth. You can also totally buy this puree frozen at your local grocery store - just prepare it according to package directions and let cool before using.
The Chai Seasoning makes about 2/3 cup of seasoning. It will last for months in your pantry in an airtight container. If you do not want to make this much the ratio I used is roughly: 2 parts cinnamon, 1 part ginger, 1 part cardamom, 1/2 part ground cloves, 1/2 part ground nutmeg, 1/2 part allspice.