Special equipment: a 9-by-5-inch loaf pan
For the marmalade butter: Mix the butter, marmalade, orange zest and sea salt together in a bowl until smooth. Cover and refrigerate for at least 30 minutes to allow the flavors to meld. Remove from the refrigerator 15 minutes before serving.
For the swirl bread: Preheat the oven to 350 degrees F. Brush a 9-by-5-inch loaf pan with butter, or spray with nonstick spray; set aside.
Combine the flour, baking soda, pumpkin pie spice, baking powder and fine salt in a medium bowl.
Place the sugar and oil in the bowl of a stand mixer fitted with the paddle attachment and beat on medium-high speed until well combined (it'll look like wet sand), about 1 minute.
Turn the mixer to medium speed and add the eggs, one at a time, beating well after each addition. Reduce the speed to low, add the pumpkin puree and vanilla extract, and mix until just combined, about 30 seconds. Stop the mixer and scrape down the sides of the bowl and the paddle. Add the flour mixture, and mix until just combined.
Remove the bowl from the mixer and drizzle the melted chocolate evenly over the batter.
Using a rubber spatula, fold the chocolate into the batter until it's just swirled in. (Be careful not to fully incorporate the chocolate into the batter--you want a marbled effect.)
Pour the batter into the prepared loaf pan and spread it into an even layer. Bake until the bread is browned on top and a toothpick inserted into the center comes out with a few moist crumbs attached, 50 to 60 minutes. Let cool in the pan on a wire rack for 10 minutes. Remove from pan and let cool another 10 minutes before slicing. Serve with the marmalade butter.