Preheat oven to 375 degrees.
Boil elbow macaroni until "al dente" by following instructions printed on the package. Avoid overcooking until mushy because it will negatively impact the taste and texture of this side dish.
Pour cooked/drained elbow macaroni in an ungreased 9 x12 casserole dish.
Sprinkle with salt, coarse ground black pepper, and garlic powder (if using). Stir until well combined. Set aside.
Prepare the cheese. Thinly slice or shred ONE block of cheddar cheese and ONE block of Monterey Jack cheese. Layer slice cheese over top of seasoned elbow macaroni. Set aside.
In a medium sized bowl or large measuring cup, beat one egg. Add buttermilk, sour cream and beaten egg. Stir together until well combined.
Drizzle liquid mixture over top of sliced cheese. Stir gently until macaroni and cheese are both coated.
Place in preheated oven for 12-15 minutes OR until cheese is melted.
While dish is in the oven, cut remaining block of cheddar cheese into thin slices.
Carefully remove hot casserole dish from oven. Stir mixture until melted cheese is evenly distributed.
Layer remaining sliced cheese over top. Place under broiler for 5 to 10 minutes OR until cheese has melted and covered with brown spots.
After baking, Homemade Macaroni and Cheese may be slightly soupy. After cooling 10-12 minutes, it will thicken into a creamy cheese sauce.
Serve hot.