Boil a large pot of salted water and add elbow macaroni. Cook until al dente, about 7–8 minutes. During the last 3 minutes, add the diced Yukon gold potatoes. Drain and set aside.

Cook the bacon in a skillet until crispy. Transfer to a paper towel-lined plate to cool, then crumble into bite-sized pieces. In the same skillet, melt 1 tablespoon of butter and toast the breadcrumbs until golden brown. Set aside.

Melt 4 tablespoons of butter in a large saucepan. Whisk in the flour and cook for 1–2 minutes to form a roux. Gradually whisk in the milk, bringing it to a gentle simmer. Stir in the garlic powder, salt, and pepper, then add the sharp cheddar and Monterey Jack cheeses until melted. Fold in the sour cream until blended.

Combine the cooked pasta and potatoes with the creamy cheese sauce, stirring gently. Pour into a greased 9x13-inch baking dish. Top with the toasted breadcrumbs and crispy bacon.

Bake in a preheated oven at 375°F (190°C) for 20–25 minutes, until the top is bubbly and golden brown. Sprinkle with chopped green onions before serving.