Preheat & prep. Preheat oven to 375°F (190°C). Line 4 muffin wells with liners or grease them. (If you only have a full 12-cup tin, fill 4 wells and leave the others empty.)
Combine dry ingredients. In a medium bowl whisk together flour, packed brown sugar, baking powder, baking soda, cinnamon, nutmeg (or pumpkin pie spice), and salt until evenly mixed.
Whisk the wet. In a separate bowl whisk pumpkin purée, oil (or melted butter), egg (use ~2/3 if measuring; using 1 whole egg is fine), and vanilla until smooth.
Fold together. Pour wet mixture into dry. Gently fold with a spatula until just combined — a few streaks of flour are okay. Stir in pecans or chocolate chips if using. Do not overmix.
Fill tins & bake. Divide batter evenly among the 4 prepared wells (about two-thirds full). Tap the tin lightly on the counter to release big air bubbles. Bake 18–22 minutes, checking at 16 minutes; muffins are done when a toothpick comes out with a few moist crumbs but no wet batter.
Cool. Let muffins cool in the tin for 5 minutes, then transfer to a cooling rack. Serve warm or at room temperature.
