1. Preheat oven to 400ºF.

2. In a medium bowl, combine flour, baking powder, baking soda, cinnamon, and nutmeg. Whisk together and set aside.

3. In a large mixing bowl, beat together butter, sugar, and brown sugar until creamy. Add vanilla and eggs, one at a time, beating between additions. Add pumpkin and yogurt, beat well.

4. Add flour mixture a little at a time, beating between each addition. Fold in walnuts.

5. To prepare the streusel topping, combine flour, brown sugar, and cinnamon in a bowl and mix together with a fork. Add butter and combine until mixture resembles coarse meal. (Using your hands is useful here.)

6. Pour batter into 12 muffin cups until ¾ full. Sprinkle each with streusel topping. Bake for 20 minutes. Remove from oven and cool.