Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a light-colored saucepan, melt the unsalted butter over medium heat. Continue cooking, stirring occasionally, until the butter turns golden brown and nutty-smelling, with brown bits accumulating at the bottom (about 5-8 minutes). Pour the brown butter into a large heatproof bowl and let it cool for 10 minutes.
Step 3: To the cooled brown butter, add the granulated sugar and brown sugar. Whisk until well combined and smooth.
Step 4: Add the pumpkin puree, vanilla extract, and egg yolks to the butter-sugar mixture. Whisk vigorously until the mixture is smooth and fully incorporated.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, pumpkin pie spice, cinnamon, baking powder, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Be careful not to overmix.
Step 7: Fold in the chocolate chips or chopped nuts, if using.
Step 8: Pour the batter into the prepared baking pan and spread evenly. Optionally, sprinkle a few extra chocolate chips on top.
Step 9: Bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs (not wet batter). Do not overbake.
Step 10: Let the blondies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Cut into squares and serve.