How to Make It
Place potatoes in a large pot, and add cold water to cover by 2 inches. Bring to a boil over high. Reduce heat to medium-high, and simmer until potatoes are tender when pierced, 35 to 40 minutes.
While potatoes simmer, cook poblano directly over a medium flame of a gas stovetop, or under broiler, turning occasionally, until skin is charred and blistered, 11 to 12 minutes. Place chile in a small bowl, and cover with plastic wrap; let steam 20 minutes.
Drain potatoes, and let cool slightly, about 20 minutes. Peel potatoes with a sharp paring knife; transfer to a large bowl, and mash until smooth.
Gently scrape off poblano skin with paring knife. Cut in half lengthwise; remove and discard seeds. Finely chop poblano, and place in bowl with potatoes.
Melt butter in a large, high-sided skillet over medium-high. Add mushrooms; stir to coat, and cook, stirring occasionally, until mushrooms are tender and liquid has evaporated, 5 to 6 minutes. Add mushrooms, salt, and black pepper to potato mixture, and fold until thoroughly blended.
Follow assembly and frying instructions at left using tortillas and oil. Serve taquitos hot with Mother Salsas and cooked rice.