Heat the oven to 350°F. Grease a standard 12-cup muffin pan, or line it with paper or foil liners.

In a large bowl, thoroughly whisk together the flour, granulated sugar, brown sugar, pumpkin pie spice, baking soda, baking powder, and salt, breaking up any brown sugar lumps.

In a second bowl, whisk the eggs, pumpkin, and melted butter until smooth and well blended.

Set aside a small handful of chocolate for the tops. Stir the rest of the chocolate and the walnuts into the pumpkin mixture.

Combine the wet and dry mixtures and fold with a rubber spatula just until no dry flour remains. The batter will be thick and lumpy; don't overmix.

Divide the batter evenly among the 12 cups. They will be full, mounded over the rims. Gently press the reserved chocolate into the tops.

Bake for 20 minutes (up to 25 if needed), until the muffins are puffed and spring back when gently pressed, and a toothpick in the center comes out with moist crumbs but no wet batter.

Cool in the pan for a few minutes, run a butter knife around each muffin, and lift them onto a wire rack to finish cooling.