Heat a barbecue or gas stovetop to medium-high and place poblanos on grill or directly on gas flame. Cook, flipping frequently, until charred and blistered on all sides and softened. Let cool slightly, then place in a large resealable bag while still hot and set aside to sweat for 10 minutes.

Peel off blackened skin. Cut lengthwise; open, then cut away seeds and stems and discard. If using canned poblanos, drain, then remove seeds and stems. Slice the chilies lengthwise into 1⁄4-inch-thick strips.

Melt butter in large frying pan over low heat. Add onion and garlic and cook, stirring, until softened, 5 minutes. Add poblanos, corn kernels, most of the sour cream, white pepper and salt and cook, stirring, until heated through, 5 minutes.

Serve the poblano mixture on warmed tortillas with Mexican rice and a drizzle of the remaining sour cream.