Punching power comes from the ground up: Leg drive, hip and trunk rotation, and a relaxed arm that accelerates late. Build rotational power with medicine ball throws, develop hip and leg strength, and refine technique so the sequence stays connected.
A punch is a sequence, not an arm action. Force begins with the rear foot pushing into the floor, travels through hip rotation, is transmitted by a stiff trunk, and arrives at the fist last. The arm contributes relatively little on its own. This is why heavier lifters do not automatically punch harder and why technically sound fighters often out-hit stronger ones.
Leg drive requires leg strength. Back squats, hip thrusts, and split squats give you something to push the floor with. Add push presses or trap bar jumps for whole-body triple extension and speed.
Tension in the shoulder and arm slows the punch and reduces the force arriving at the target. Fighters are taught to stay loose and tighten only at impact for exactly this reason. Shadowboxing with an emphasis on staying relaxed until the final instant is genuine power training.
A punch that stays extended is a punch you pay for. Practise snapping the hand back to guard, which also keeps the shoulder healthier over time.
Two power sessions a week, early in the session, with low volume and full recovery between efforts. Keep the heavy strength work away from your most important sparring days, and remember that technical coaching usually adds more punching power than any exercise. Background on the transfer from strength to speed is in Power Development and Explosive Strength.