Building up Wrists

i have been building myself up for the last 5months, and i'm seeing some really good resuts, or atleast i think so
My shoulders are getting bigger as are my biceps forearms and chest. But my wrists are getting me down , getting worried that they are going to stay skinny

What exercises are there that can build up my wrists?

Wrist flexion and wrist extension are two lifts that will work all of the muscles in the lower arm.

Wrist flexion:
Grasp a weighted bar bell, palms up with thumbs under the bar. Place hands 4-6 inches apart, sit on a bench and put the backs of your forearms on top of your upper legs. Forearms should be parallel to the floor, lean forward so the angle between the upper and lower arms is 90 degrees or less. Extend the wrists over the knee caps, then pull/curl the bar up as high as possible by bending the wrists, pause for 1-2 seconds, and lower the weight in a controlled manor (4 seconds) back to the start position. Do not jerk or bounce the weight and do 1 or 2 sets of 8-12 reps.

Wrist extensions:
Using dumbbells work one arm at a time. Sit on a bench like described above only place the front/palm side of the forearm down on the upper leg instead of the back of the forearm. Extend the wrist up as far as possible by bending the wrists, pause, and slowly lower the weight like previously described.

that should work for a normal person, but if you're an ectomorph with thin wrists, you'll never be able to get them the way you want. You can't make your bones bigger or produce more flesh around them

Your wrists are subject to your genetics. If you want to build your forearms (& grip, traps, core, etc) try some Farmers Walks.