Combine butter, sugars, and pumpkin pie spice in a medium mixing bowl and beat with an electric mixer until creamy.
Add pumpkin and egg yolk and beat until smooth.
Mix in flour and baking powder and beat on low speed until combined; stir in oats and white chocolate chips.
Drop dough by 1/8 cup (2 Tbsp) 2 inches apart on ungreased cookie sheet (*cookies are easiest to remove if sheets are lined with parchment paper.)
Bake at 350 degrees 16-20 minutes or until edges are golden brown.
Cool on cookie sheet 10 minutes; remove to a wire rack to cool completely.