Place melted butter, pumpkin puree, brown sugar, and granulated sugar in the bowl of a strand mixer fitted with the paddle attachment. (You can also use a hand mixer.) Mix until combined. Continue mixing and add the egg yolk and vanilla extract. Add the baking soda, nutmeg, cinnamon, ginger, and allspice and mix until combined, scraping the sides of the bowl as needed. Slowly mix in flour until a smooth dough forms. Press dough evenly into the bottom of the pan.