Preheat the oven to 350°F degrees and line a large baking sheet with parchment paper.
First, brown the butter. Add butter to a small saucepan over medium-high heat and cook, stirring often, until it foams, then browns and small brown flecks appear, roughly 5 to 8 minutes. Once brown, pour into a large bowl, making sure to scrape the bottom of the pot for all of the brown bits, then transfer to the fridge to chill for 20 minutes.
Next, dry your pumpkin. Line a small plate with a tea towel and a couple of sheets of paper towel on top. Dollop the pumpkin puree onto the paper towel, spreading it out a little bit, set it aside, and allow to rest while your butter cools. The tea towel and the paper towel will absorb the excess moisture in the pumpkin puree helping to produce a crisper and chewier cookie. Skipping this step will make the cookies softer and create a more muffin-like or cake-like texture.
In a small bowl, add flour, rolled oats, baking soda, salt, cinnamon, ginger, nutmeg, cloves, and allspice, mix with a spoon or spatula until well combined.
Once the butter has cooled, remove the bowl from the fridge, add brown sugar and whisk until smooth and well combined.
Once the butter and sugar are mixed, add “dried” pumpkin puree (gently scrape it off the paper towel), maple syrup, vanilla extract, and egg yolk, and mix again until well incorporated.
Add the dry ingredients to the wet ingredients and gently mix until well incorporated, being careful not to over mix the batter.
Using a 2-ounce cookie scoop or 1/4 cup measuring spoon, portion the dough onto the baking sheet, leaving roughly 3 inches between them. You should be able to fit 5-6 cookies on the baking sheet at a time. Place the bowl of extra dough in the fridge while the first batch of cookies bakes.
Transfer the baking sheet to the oven and bake for 14-16 minutes, until the edges are slightly golden and the centre still looks a little soft. Continue until all of the dough has been used.
Once cooked, remove the baking sheet from the oven and allow the cookies to cool a few minutes on the baking sheet, then transfer them to a wire cooling rack to cool completely. (If you have two baking sheets I would recommend using the second baking sheet to bake the next batch to give the cookies more time to rest before moving them to the cooling rack.)
Once cooled, the cookies can be served immediately, stored in an airtight container for up to 5 days, or in the freezer for up to 3 months.