Melt the butter and chocolate in a double boiler or in the microwave in 30 second increments until just melted.
In another bowl, stir together the sugar, eggs and vanilla extract (only the amounts listed under brownie batter above). Once the chocolate/butter has cooled some, stir the chocolate mixture in with the egg/sugar mixture. You don’t want the chocolate mixture to be too hot here because eggs have a tendency to very quickly respond to heat. So, if the chocolate mixture is piping hot, it could cook your eggs and create a funky texture in your brownie batter, which you don’t want.
Then stir in the flour, baking powder and salt.
In a small bowl, stir together the pumpkin batter ingredients listed above. Pour 3/4 of the brownie batter into a square baking pan that has been lined with parchment paper. Spoon the pumpkin batter over the top of this. Then add the remaining brownie batter and swirl with a butter knife.
Bake at 350°F for 28-30 minutes. A toothpick inserted anywhere other than the center should come out clean, while the batter may still be just a little undercooked in the center. Again, this is just my preference.Allow to cool before removing from the pan and slicing into squares.