Preheat the oven to 400. In the top of a double boiler, or metal bowl set over (but not touching) gently simmering water, add the butter and the chocolate. Stir occasionally until just melted. Remove from heat, stir in 1 cup pumpkin porter and vanilla extract.
In the bowl of a stand mixer beat the eggs on high until light and frothy, about 2 minutes. Add both kinds of sugar and beat for 6 full minutes. Add the pumpkin puree, stir until combined.
In a separate bowl add the flour, cinnamon, espresso powder, salt and cocoa powder, whisk until well combined.
While the mixer is on low, add the chocolate mixture to the eggs. Mix until well incorporated, stopping to scrape the bottom of the bowl to insure the batter is fully combined.
Remove the bowl from the stand mixer, sprinkle with dry ingredients. Stir until just combined.
Grease a 9x13 baking dish, or spray with butter flavored cooking spray, pour in batter.
Place in the oven and immediately reduce to 350. Bake at 350 for 35 minutes. The top should look completely dry but the center should still be fudgy. Don’t over bake. Remove from oven and allow to cool until set and come to room temperature before attempting to cut, about 1 hour.
Add the dark chocolate, corn syrup, and 1/3 cup pumpkin porter to the top of a double boiler over medium heat.
Stir until melted and well combined, remove from heat, pour over ice cream.