For the brownie batter: Melt the butter and chocolate together in a medium saucepan over low heat until smooth, stirring constantly with a wooden spoon. Remove from heat. Stir in the brown sugar and 1 teaspoon vanilla, then with a whisk, beat in 4 eggs, one at a time, mixing well after each. With a wooden spoon, stir in 1 1/2 cups flour, baking powder and salt until no floury streaks remain.
Spread half of the brownie batter into the baking pan. Drop alternating spoonfuls of the pumpkin batter and the remaining brownie batter into the pan, then sprinkle the toasted pecans over all.
Run a table knife through the batter in a zigzag motion to make attractive swirls. Don't overdo it or the colors will lose their definition.
Bake 35 to 37 minutes until the edges are firm and the middle is just set. Let cool completely on a wire rack, then cover well and chill overnight in the refrigerator.
The next day, use the "handles" of the parchment or aluminum foil to lift the bars from the pan. Peel off the paper, then slice.
Nutrition information per each of 48 bars: