In a large bowl, use the paddle attachment of an electric mixer to cream together the butter, sugar, egg yolk, vanilla, and pumpkin on medium low speed until incorporated.
In a separate bowl, use a hand whisk to gently whisk together the oats, flour, salt, baking powder, baking soda, pumpkin pie spice, and allspice. Use a rubber spatula to fold the dry ingredients into the wet ingredients, just until no dry flour streaks remain (don't over mix.) Fold in the white chocolate chips and pecans. Cover and chill dough at least 1 hour or until firm enough to handle.
Preheat oven to 350F with rack on lower middle position. Line baking sheets with silicone liners or parchment paper. Form rounded 1 TB dough balls and place at least 2 inches apart on lined baking sheets. Flatten dough balls to the shape of a cookie, as these will not spread out too much in oven. Bake about 9 minutes, or just until edges are lightly browned and center seems slightly undercooked. Cookies will set upon cooling. Let cookies cool about 5 minutes before carefully transferring to wire rack to finish cooling.