Preheat oven to 350 degrees. Butter and flour a 5- by 9-inch nonstick loaf pan.
In a mixing bowl, combine flour, baking powder, baking soda, pumpkin pie spice and salt, and stir together.
In a large bowl, with electric mixer, beat butter until softened. Add sugar and continue beating. Add eggs and beat until well blended. Add pumpkin, vanilla and finally the flour mixture, beating just until well-blended.
Melt chocolate in a double boiler over medium-high heat until totally melted, about 5 minutes. Or melt in a glass bowl in microwave for about 1 1/2 to 2 minutes, or until completely melted and smooth.
Spoon half the batter into prepared loaf pan. Drop spoonfuls of half the chocolate on top of batter and then swirl chocolate into batter with a wooden skewer. Repeat with remaining batter and chocolate, making sure to swirl chocolate into pumpkin well (but leaving very visible streaks).
Bake loaf in preheated oven about 1 hour or until skewer inserted into center comes out clean. Let loaf cool at least 15 minutes, then invert onto a wire rack. Serve warm or at room temperature, sliced.