Crumble chorizo in a large skillet, and cook over medium-high, stirring occasionally, 5 minutes. Stir in onion and garlic, and cook, stirring occasionally, until onion is tender, about 5 minutes. Add ground chuck, chili powder, and 1/2 teaspoon of the salt; cook, stirring often, until beef is browned and done, about 10 minutes. Remove from heat, and stir in cilantro.
Using a slotted spoon, divide chorizo-beef mixture evenly among tortillas (about 1/4 cup per tortilla), placing mixture just below center of each tortilla. Roll tortilla over filling, leaving about 1 inch of tortilla exposed. Brush exposed edge with a small amount of beaten egg; press gently to seal roll.
Heat oil in a large skillet over medium; fry flautas, in batches, until crispy, 3 to 4 minutes per side. Drain on paper towels.
Process avocado, sour cream, buttermilk, lime juice, and remaining 1/2 teaspoon salt in a food processor until smooth. Serve flautas with crema.