Place butter in a small saucepan and melt over low heat. Once melted, whisk in the miso paste until smooth — it will resist at first, but keep whisking until completely incorporated. Stir in lemon zest and juice, then add 1-2 tablespoons water to thin the sauce to a pourable consistency. Set aside off heat.
Heat a large skillet over medium-high heat until hot. Add the shrimp in a single layer and cook without moving them until they begin to curl and turn pink on the bottom, about 2-3 minutes. The undersides should have some color.
Pour the reserved miso butter sauce over the shrimp and toss to coat. Continue cooking, stirring frequently, until the shrimp are completely opaque and cooked through, another 1-2 minutes. The sauce should cling to the shrimp.
Transfer to serving plates immediately. This pairs beautifully with steamed white rice to soak up the sauce, or toss with hot pasta for a more substantial meal.