1/2 cup (1 stick) unsalted butter (softened, plus more for greasing the pan)
1 cup (packed) dark brown sugar
2 large eggs
1/2 cup unsulfured molasses
1/2 cup unsweetened pumpkin purée
2 cups whole wheat pastry flour
2 teaspoons baking powder
2 teaspoons baking soda
1 tablespoon ground ginger
1 1/2 teaspoons ground cinnamon
1/2 teaspoon salt
1/3 cup boiling water
  Whipped cream (as garnish)
1/4 cup finely minced crystallized ginger (as garnish)