Preheat the oven to 325°F Line an 11- by 18-inch (rimmed) baking pan (hs note: or jellyroll pan) with parchment paper and lightly oil with canola oil spray.
Melt the chocolate and butter in a glass bowl in the microwave for 1 1/2 to 2 minutes on high. Stir with a spoon to melt the chocolate completely. Place the beans, 1/2 cup of the walnuts, the vanilla extract, and a couple of spoonfuls of the melted chocolate mixture into the bowl of a food processor. Blend about 2 minutes, or until smooth. The batter should be thick and the beans smooth. Set aside.
In a large bowl, mix together the remaining 1/2 cup walnuts, remaining melted chocolate mixture, coffee substitute, and salt. Mix well and set aside.
In a separate bowl, with an electric mixer beat the eggs until light and creamy, about 1 minute. Add the agave nectar and beat well. Set aside.
Add the bean/chocolate mixture to the coffee/chocolate mixture. Stir until blended well.
Add the egg mixture, reserving about 1/2 cup. Mix well. Pour the batter into the prepared pan. Using an electric mixer, beat the remaining 1/2 cup egg mixture until light and fluffy. Drizzle over the brownie batter. Use a wooden toothpick to pull the egg mixture through the batter, creating a marbled effect. Bake for 30 to 40 minutes, until the brownies are set. Let cool in the pan completely before cutting into squares. (They will be soft until refrigerated.).