Bring the brown sugar, rum and orange juice to a low boil and, stirring often, boil for about 8 minutes.While the cake is still warm, set the rack above a plate and brush on the glaze about 3 times over the course of 15 minutes, allowing syrup to be absorbed. Return any glaze that runs onto the plate back to the cake. Wipe the plate clean and press the reserved one cup walnut-orange zest mixture evenly over the cake. Press any walnuts that fall onto the plate back on the cake. Cool completely and serve.