Melt the butter and set aside to cool slightly.
Preheat the oven to 350°F (176°C) and line a muffin pan with paper liners.
In a medium bowl, whisk together the flour, baking powder, salt and spices.
In a large bowl, whisk together the pumpkin, brown sugar, vegetable oil, eggs and vanilla until smooth.
Now whisk the melted butter into the pumpkin mixture.
Whisk the dry ingredients into the pumpkin mixture until just combined and most of the lumps are gone. Be careful not to overmix.
Divide the batter evenly among the 12 baking cups. They should be about 3/4 full. Gently smooth the tops with the back of a spoon.
Bake for 22 to 26 minutes, or until a toothpick inserted in the center comes out clean. Let the cupcakes rest in the pan for 5 minutes before transferring to a wire cooling rack.