Preheat the oven to 170°C / 340°F (fan). Grease and line a 22×10 cm loaf pan with parchment paper.
Combine all dry ingredients in a bowl (flour, baking soda, baking powder, salt, and spices).
In a large bowl, whisk together melted butter, oil, and sugars until smooth.
Add eggs one at a time, whisking gently after each addition.
Stir in pumpkin purée, yogurt, and vanilla, mixing gently until the batter is silky.
Fold in the dry ingredients with a spatula until just combined. (Add walnuts or chocolate if using.)
Pour the batter into the prepared pan, sprinkle brown sugar on top, and bake 55–65 minutes, or until a toothpick comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack. Once fully cool, wrap tightly - it becomes even softer the next day.