Clean and trim mushrooms, removing stems if needed.
In a bowl, whisk together flour, breadcrumbs, paprika, garlic powder, onion powder, salt, and pepper.
In another bowl, whisk the egg and buttermilk.
Dip each mushroom in the buttermilk mixture, then coat thoroughly with the flour mixture.
Heat oil in a deep skillet or fryer to 350°F (175°C).
Fry the mushrooms in batches until golden brown, about 4-5 minutes per batch. Remove and drain on paper towels.
For the chipotle sauce, blend mayonnaise, sour cream, chipotle peppers, lime juice, and salt until smooth.
Serve the fried mushrooms with the chipotle sauce on the side for dipping.