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.)