Two of the best exercises for working the brachialis and building nice bicep peak are the overhead cable curl from the lat pull down machine. And the reverse grip low pulley cable curl.
These exercises really target and isolate the biceps and help develop more muscle fullness and shape. The brachialis muscle is right under the biceps and when it is fully developed it will help push the biceps up higher adding to the overall muscle peak.
Do these exercises at the end of your bicep workouts as a finishing movement. Start with a heavy power exercise first, such as the standing barbell curl or dumbbell curl, and then move on to the overhead cable curls, and then do the low pulley cable reverse curls. 3-4 sets of about 10-15 reps per set will really give your biceps a great muscle pumping workout.