Directly over the flame of a gas burner, or in a cast iron pan set over high heat, char the poblano until the skin is completely blistered. Place pepper in a zip top bag and allow to steam for 5-10 minutes. Meanwhile, in a small bowl stir together crema, lime zest, and salt. Once pepper has steamed, remove skin and seeds and finely chop. Stir into crema mixture and set aside.
In a small airtight container, stir together spices. Place shrimp in a large bowl. Toss with 2 T. spice rub and set shrimp aside to marinate 10-15 minutes. Store remaining spice rub for another use.
Meanwhile, in a large bowl whisk together lime juice and olive oil. Add cabbage, cilantro leaves, and scallions to the bowl and toss to thoroughly combine. Season with salt to taste and set aside.
Heat a grill pan over high heat. Lightly brush with oil and grill shrimp, turning once, about 2 minutes per side. Toast buns alongside shrimp, about 1 minute, until desired color is achieved.
To serve, spread poblano crema on top and bottom of buns. Divide shrimp among sandwiches and top with cabbage slaw and cilantro leaves.