How to Make It
Heat oven to 350° F. Brush an 8 1/2-by-4 1/2-inch loaf pan with butter and dust with flour, tapping out the excess. Combine the chocolate and 2 teaspoons of the oil in a medium microwave-safe bowl. Microwave on high in 30-second intervals, stirring in between, until melted. Set aside to cool to room temperature.
Whisk together the flour, cinnamon, baking soda, baking powder, and salt in a medium bowl; set aside.
Beat the butter, sugar, and the remaining 1/4 cup oil in a large bowl with an electric mixer on medium-high until light and fluffy, 1 to 2 minutes. Add the pumpkin puree and mix until combined. Add the eggs, one at a time, and vanilla and mix until combined. Reduce mixer speed to low and slowly add the flour mixture. Mix until just combined.
Transfer half the batter into the prepared pan. Top with two-thirds of the chocolate mixture. Repeat with the remaining batter and chocolate mixture. Using a skewer or a knife, swirl the chocolate and batter together to create a marbled pattern.
Bake until a toothpick inserted in the center comes out clean, 50 to 60 minutes. Cool in the pan for 15 minutes. Transfer to a wire rack and let cool completely.