Preheat oven to 350 degrees. Spray a 13x9 pan with cooking spray.
In the bowl of a stand mixer, cream together butter and brown sugar until light and creamy, about 3 minutes.
Mix in egg yolk, vanilla and pumpkin puree.
Sprinkle baking soda, baking powder, salt and pumpkin pie spice over the top of the wet ingredients. Mix briefly.
Add flour and oats in alternating 1 cup amount, starting and ending with flour. Mix in between. (1 cup flour. Mix. 1 cup oats. Mix. Flour. Mix. Oats. Mix. Flour. Mix.)
Stir in M&ms, chocolate chips, dried cranberries and chopped walnuts.
Spread cookie dough into sprayed 13x9 pan. Bake at 350 for 30 minutes, or until edges are browned and the center is set.
Remove and let cool completely before cutting into 24 bars.