Grease a bundt tin with a generous amount of butter and then add most of the the crushed Doritos, reserving a handful for later. Move the tin around so that they stick to the butter and coat the whole inside of the tin. Set aside.
Cook the macaroni in a large saucepan of boiling salted water for 8–10 minutes. Drain well and set aside.
Preheat the oven to 180 C.
Melt the butter over a medium heat in a large saucepan. Add the flour and stir to form a roux, cooking for a few minutes.
Gradually whisk in the milk, a little at a time. Cook for 10–15 minutes to a thickened and smooth sauce.
Remove the sauce from the hob, add the cheddar and the fontina and stir until it is well combined and melted. Take off the heat and then add the eggs and stir to combine.
Add the macaroni to the sauce and mix well. Transfer half to the to the bundt tin and press down well.
Add slices of the cooking mozzarella around the whole of the bundt tin and then top with the remaining pasta and sauce.
Pack down and flatten with a spatula or the back of a spoon then sprinkle over the mozzarella and Doritos and place the dish in the oven for 25 – 30 minutes.
Cook until the cheese is browned and bubbling. Leave to cool for 10 – 15 minutes then remove the tin and serve in slices.