Place the butter in a light-colored saucepan over medium heat. As it melts, it will begin to foam and sizzle. Swirl the pan occasionally and watch carefully as the butter turns from lemony yellow to golden and finally to amber with brown bits at the bottom—about 5-7 minutes. Once it smells like toasted hazelnuts and has amber specks, immediately transfer to a heat-proof bowl to stop the cooking. Let cool for 10 minutes.
Preheat your oven to 350°F (175°C). Line a 9×5-inch loaf pan with parchment paper, leaving an overhang on the sides for easy removal.
In a medium bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, cloves, and salt.
In a large bowl, combine the pumpkin puree, granulated sugar, and brown sugar. Mix until smooth. Add the cooled brown butter (including all those flavorful browned bits!), eggs, and vanilla extract. Whisk until completely incorporated and the mixture looks silky.
Add the dry ingredients to the wet ingredients in two batches, alternating with the buttermilk. Mix gently with a spatula just until combined—overmixing develops gluten, which will make your bread tough. If using nuts or chocolate chips, fold them in now.
Pour the batter into your prepared loaf pan and smooth the top. If using, sprinkle pepitas on top. Bake for 55-65 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs attached.
Allow the bread to cool in the pan for 15 minutes before using the parchment overhang to lift it out. Let it cool completely on a wire rack before slicing.