Measure the pumpkin purée, then blot it between paper towels to remove excess surface moisture.
Whisk the flour, pumpkin pie spice, cinnamon, baking soda, and salt in a large bowl.
In a second large bowl, cream the softened butter, brown sugar, and granulated sugar with a stand mixer and paddle attachment or an electric hand mixer for 1–2 minutes.
Mix in the egg yolk and vanilla, followed by the blotted pumpkin.
Mix in the dry ingredients, then add the rolled oats. Stop once the dough is evenly combined.
Cover the bowl and refrigerate the dough for 30 minutes.
Heat the oven to 350°F (177°C). Line two large baking sheets with parchment paper.
Scoop level 1½-Tbsp portions, roll them into balls, and lightly flatten each one.
Bake one sheet at a time for 9–12 minutes. Remove the cookies when the edges are lightly browned, the tops are set, and the centers remain soft.
Cool on the baking sheet for 5 minutes, then transfer the cookies to a wire rack to cool completely.