Preheat the oven to 375 F and prepare your muffin pans. Either spray with non-stick oil or use shortening and flour.
Mix together the dry ingredients, flour, baking powder, baking soda, salt and nutmeg in large bowl and set aside.
Cream the butter with the brown sugar and white sugar for 3 minutes in a mixer with a paddle attachment, until it's light and fluffy. Add in the eggs and beat until combined. Reduce the speed to low and add in ⅓ of the flour mixture with ½ of the yogurt.
scrape down the sides of the bowl and add another ⅓ of the flour and the remaining yogurt. Scrape down the sided and add in the remaining flour, mixing until just combined.
By hand, fold in the banana and walnuts.
For the Crumble
Mix all ingredients (excludiing the chocolate chips) together with two forks until large, buttery crumbs form.
Fill your muffin tins ¾ full with the batter and top with a pinch of crumble topping. Bake for 25 - 30 minutes. They are done when they are golden brown and you can test with a toothpick inserted in the center comes out with a few crumbs attached.
Add some mini chocolate chips to the muffins right out of the oven. Leave in the pan for 5 minutes, then cool on a wire rack before serving.