Anytime I attempt to pan-sear shrimp, I struggle to achieve a perfect char. I typically turn the stove on high, pre-heat a stainless steel pan, use butter + olive oil, and then add the shrimp for a couple minutes each side. I also use a fish spatula to make sure they're flat to the pan. I find after a few minutes each side, the shrimp are done but not charred or crisp, but if cooked much longer, the shrimp become too well done. Do certain pans help accomplish this? Does type of shrimp matter? Is oil / butter preferred? What is the proper way to effectively pan-sear shrimp? 
There are a few things you can do to help your shrimp get a nice sear. First, make sure they're as dry as possible before adding them to the pan. Use paper towels and pat dry. Especially if you're using shrimp which you had to thaw, they can be pretty wet, and that'll cause them to steam instead of sear. Another thing which can cause your meat to steam instead of sear is if you overcrowd the pan. Use a larger pan or do less shrimp at once. When I cook shrimp, I can see at least 2/3 of the bottom of the pan through the shrimp. If all you can see is shrimp, it's way too much, and the water it releases will get trapped instead of quickly evaporating off. Getting your shrimp to room temp can also help, as the colder your meat is when you put it in the pan, the more your pan will cool down when you add it. A thicker pan is another solution, as it has more capacity to hold heat. Cast iron is ideal for searing for this reason. Adding less shrimp at once will also reduce this effect. By using the fish spatula, do you mean you're pressing down on them while they're cooking? I'd skip this, it can squeeze out liquids, so it might hurt more than help. Shrimp are pretty flat, so just toss them in the pan and then leave them alone until they need flipped. Lastly, I might skip the butter and only use an oil which can tolerate high heat. Butter will burn at high temps, so you have to keep your pan cooler, which means less of a sear. Your butter/oil should not be smoking at any point, that means the temp is too high for that type of oil. Shrimp in butter does taste better though IMO, so you can try changing this as a last resort. 