Preheat the oven to 350ºF (180ºC).
Line a 9x13 inch pan with parchment paper (bottom and two sides only, it will be easier to unmold), and spray or butter the unlined sides.
In a large bowl, beat 1 cup butter adding 2 cups light brown sugar gradually until very creamy, 2 to 3 minutes.
Add 2 eggs, one at a time, and beat well. Add 1 teaspoon vanilla extract and mix well.
Sift 2 cups all-purpose flour with 2 teaspoons baking powder, 1 teaspoon ground cinnamon, ½ teaspoon ground ginger, ½ teaspoon ground allspice and ½ teaspoon salt and add it in 2 parts to the butter mixture. Mix only until starting to blend.
Add 1 ½ cups pumpkin puree and mix well. Pour into the prepared pan, spreading evenly.
In a small bowl mix ½ cup sour cream (or cream cheese) with ¼ cup white sugar and 1 tablespoon heavy cream.
Drizzle on top of the pumpkin batter. Sprinkle 1 cup pecans on top and bake for about 30-40 minutes, until a tester comes out clean.
Let cool on wire rack. Remove carefully from the pan. Serve at room temperature.
It keeps at room temperature for 2 days and a week in the fridge covered in plastic wrap.