Preheat oven to 175°C (350°F) and line a 9x9-inch pan with parchment paper, leaving overhang.
Whisk together flour, baking powder, salt, cinnamon, ginger, nutmeg, and cloves in a medium bowl.
In a large bowl, whisk melted butter, brown sugar, and granulated sugar until smooth. Add pumpkin puree, eggs, and vanilla extract; mix until fully combined.
Fold dry ingredients into wet ingredients gently until just combined. Do not overmix.
Fold in optional mix-ins, reserving a few to sprinkle on top if desired.
Spread batter evenly in the prepared pan and smooth the top. Tap the pan lightly to remove air bubbles.
Bake for 30–35 minutes, until edges are set and a toothpick inserted into the center comes out with a few moist crumbs. Do not overbake.
Cool completely in the pan on a wire rack for at least 30 minutes before slicing into 16 squares.