In a large bowl, use an electric mixer to cream together butter, sugar and brown sugar. Add egg and vanilla; beat until fluffy.
Stir or whisk together flour, baking soda, salt, cinnamon, cloves, ginger and nutmeg. Stir in the oats. Add the dry ingredients to the butter mixture, alternating with the pumpkin. Fold in the optional mix-ins, if desired.
Cover and chill in the refrigerator for at least 30 minutes, or up to 1 day.
Preheat oven to 350 degrees F. Line baking sheets with parchment paper or silicone mats.
Using a 1 ½ -tablespoon cookie scoop, drop the cookie dough onto the prepared baking sheets. Use the bottom of a measuring cup or glass (or fork tines) to gently press the cookies down and flatten them slightly, because the cookies will not spread too much in the oven.
Bake for about 12-17 minutes, or until the edges of the cookies are lightly browned. You want them soft (not over-baked and dry), but the tops shouldn’t be shiny at all. Remove from the oven and cool on the baking sheet for 5 minutes, then transfer the cookies to a wire rack to finish cooling.