Bring a large pot of lightly salted water to a boil, add macaroni and cook for 8 minutes (pasta will still be slightly hard). Drain pasta and transfer to a large bowl with 65g butter and toss to coat.
Whisk together the sour cream, egg and cream cheese; add to the pasta and mix well. Stir in 3 cups of grated Asiago and 3 cups of grated Cheddar cheese reserving the remaining 2 cups of cheese for the topping.
Place the bacon in a large, deep frypan and cook over medium-high heat, turning occasionally until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate and chop into small pieces.
Preheat an oven to 200 degrees C.
Stir 40g butter, the onion and garlic into the bacon drippings in the pan and cook and stir for 2 minutes. Reduce the heat to medium-low and add the brown sugar. Cook, stirring occasionally, for 10 to 15 minutes until the onions are very soft and golden brown. Mix the cooked bacon, parsley and panko breadcrumbs into the onions, transfer mixture to a small bowl and set aside.
Return the frypan to the heat and melt remaining 2 tablespoons butter over medium-low heat. Whisk in the flour and stir until the mixture becomes paste-like and light golden brown, about 5 minutes. Gradually whisk the milk into the flour mixture and bring to a simmer over medium heat. Cook and stir until the mixture is smooth and thickened.
Stir in the ground mustard, paprika, salt, pepper and Tabasco sauce. Remove sauce from heat and let cool for five minutes.
Pour the sauce over the macaroni mixture, stirring well. Transfer to a greased 20x30cm pan and sprinkle with the remaining 2 cups of Cheddar cheese. Top cheese with the onion breadcrumb mixture.
Bake in preheated oven until bubbling, hot and golden brown on top, about 30 minutes.