Gather your ingredients
Take your crumb topping ingredients and mix them together in a large bowl. Stir with fork until pea sized crumbs form. Set aside.
Preheat the oven to 375 degrees Fahrenheit and set the oven rack to the middle position. Put your walnuts on a parchment sheet or aluminum foil & sprinkle with cayenne pepper & brown sugar. Then roast in the oven for about 7 minutes once the oven has reached 375. (Watch them!!. You're looking for the brown sugar to be slightly caramelized.) Once caramelized pull out and set aside.
Spray a 12 muffin cup tin with butter flavored Pam non stick spray.
In a medium sized bowl. Combine flour, baking powder, sea salt, 1 1/2 cups chopped walnuts and 1 1/2 cups chocolate chips. Set aside.
In a large bowl or stand mixer, beat the butter until light and fluffy, 30-60 seconds. Add in sugar and beat until dissolved (Roughly 60 seconds). Beat in one egg at a time, until combined. Stir in vanilla, sour cream, milk and mashed bananas until combined. Gently mix in dry ingredients, gentle mix until combined but do not over mix.
Fill each muffin cup to the brim. Sprinkle the top with the crumble and then sprinkle the chocolate chips that were previously set aside on the crumble.
Lastly, place a whole walnut on top for decorative purposes.
Bake for 30 minutes or until golden brown (Simple trick to check is to take a toothpick and stick it into one of the muffins. If the toothpick comes out clean. The muffins are completely cooked)
Pull the muffins out and let them cool. You can either let them cool completely or enjoy them warm.