The fastest way to improve the look of your biceps is to focus on the brachialis. The brachialis is the muscle underneath the biceps. Build it and your biceps look bigger and thicker.
The hammer curl with dumbbells, a curl where the palms are facing one another, is the usual go-to exercise for this, but replacing the dumbbells with kettlebells makes it even better.
Kettlebell Hammer Curl
This is a great biceps and biceps-brachialis exercise. It provides plenty of work for the forearms, as well as smaller muscles around the hands and wrists. It finishes with the kettlebells in an extended lever-arm position, allowing you to place constant tension on the arms throughout the movement.
What Weight to Use
You'll need to drop the weight by about half of what you'd typically use for hammer curls. But the combination of lighter loads and constant tension will leave your biceps screaming at the end of each set, producing muscular pumps and cellular swelling that's hard to get with other exercises.
Sets & Reps
Because of the lighter loads, kettlebell hammer curls work well with moderate and higher rep ranges of 8-15 reps for 2-3 sets. Just remember to keep your wrists locked in neutral position throughout.