Preheat the oven to 400 degrees F.
Toss the potatoes with the olive oil and chili powder on a rimmed baking sheet and roast until golden brown and tender, about 25 minutes.
Puree half of the avocado in a blender with the buttermilk, herbs, lemon juice, 1/2 teaspoon salt and a few grinds of pepper. Slice the remaining avocado half.
Combine the romaine with the corn, bell peppers, warm potatoes and dressing. Season with salt and pepper. Top with the chopped avocado and the tuna, broken into chunks.
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