In a medium bowl, combine shrimp, lime zest, chili powder, and 1 tsp. salt. In a separate small bowl, stir together lime juice, mayonnaise, and sour cream to create the crema, then set it aside.

Melt 2 Tbsp. butter in a large skillet over medium-high heat. Add corn in a single layer, season with 1/2 tsp. salt, and cook undisturbed for 5 minutes. Stir in the jalapeño and white scallion parts, cooking for about 3 minutes until softened. Add garlic and cook for 1 minute more until fragrant, then season with the remaining 1/2 tsp. salt. Deglaze the pan with water, scraping up any browned bits.

Push the corn mixture to one side of the skillet and melt the remaining 1 Tbsp. butter. Add the shrimp to the other side, cooking for 3 to 4 minutes total, turning halfway through, until pink and cooked through. Stir the shrimp and corn together to combine.

Remove the skillet from heat and stir in 1/2 cup of the reserved crema. Top the dish with cotija, cilantro, green scallion parts, and additional chili powder. Serve immediately with warmed corn tortillas and the remaining crema alongside.