Put the poblano peppers over the flame of 1 of the burners on the stove to roast. Cook, turning often with tongs until all of the skin is blistered, about 15 minutes. Add the peppers to a bowl and cover with plastic wrap to steam for about 15 more minutes. When the peppers are cool enough to handle, transfer to a cutting board, peel the skin off, remove the seeds and stems, dice, and set aside.
Heat 1 tablespoon of the butter in a medium saute pan over medium heat, add the onions and the salt and pepper. Cook until the onions are translucent, about 5 minutes, and then set aside to cool.
Heat the milk in a small saucepan just until it starts to simmer, about 3 minutes.
In a large bowl, whisk the eggs and then slowly add the scalded milk. Whisk in the sour cream and then fold in the cheese, corn, peppers, onions, herbs, cayenne. Season with salt and pepper, to taste.
Add the mixture to a 9 by 13-inch buttered baking dish and set aside.
Put the remaining 2 tablespoons of butter in a medium saute pan over medium heat and add the panko. Toss to coat the bread crumbs and season with salt and pepper. Pour the bread crumbs over the pudding and bake in the preheated oven until the bread crumbs are golden, the pudding is set and bubbling around the edges and a toothpick inserted into the center comes out clean, about 25 minutes. Remove from the oven and serve.