Preheat the oven to 325°F. Line an 11- by 18-inch (rimmed) baking 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.)

Keep in the refrigerator, they will slice much better if refrigerated several hours or preferably overnight.