First, preheat the oven to 375, and have a large cookie sheet ready.
In a microwave-safe bowl, add the butter. Microwave on HIGH for 40 seconds. Let the butter cool slightly, and stir with a fork to melt it fully.
To the butter, add the sugars, pumpkin, egg, and vanilla. Stir very well until combined.
In a separate bowl, whisk together the flour, oats, spices, baking soda and salt.
Add the dry ingredients to the wet, and stir until combined.
Scoop 1 tablespoon-sized balls of dough onto the cookie sheet, and press flat with your hands gently.
Bake the cookies for 12 minutes, until the edges are starting to turn golden brown. Immediately move the cookies to a cooling rack, and let cool.
They keep for up to 4 days in an air-tight container.