Preheat oven to 350°F (180°C). Line 13 x 9-in. (3-L) baking pan with parchment paper, allowing extra for overhang. Melt 3/4 cup (175 mL) butter; cool.
In large bowl, using electric mixer on low speed, beat together sugar and the 3/4 cup (175 mL) cooled melted butter. Beat in 1 egg, 1/2 cup (125 mL) flour, cocoa powder and 1/4 tsp (1 mL) salt. Scrape batter into prepared pan, spreading evenly.
In clean bowl (with clean beaters), use electric mixer on low speed to beat together remaining 1/3 cup (75 mL) butter, brown sugar, agave nectar, remaining egg and vanilla extract. Next beat in remaining 1 cup (250 mL) flour, baking soda and remaining 1/4 tsp (1 mL) salt. Fold in 3/4 cup (175 mL) chocolate nut cluster pieces. Pour over batter layer in baking pan, spreading evenly. Sprinkle with remaining 1/2 cup (125 mL) chocolate nut cluster pieces.
Bake 40 to 45 min.; covering with foil after first 15 min. Bake until golden brown on top and no longer jiggling in middle. Cool on wire rack. Using overhanging parchment, lift brookies out of pan and slice into 24 squares.
Substitute chocolate nut clusters with equal amount of white, milk or dark chocolate chips.