Melt the butter in a saucepan on a medium heat.
Add flour and mix to combine.
Add a splash of milk and combine. Continue to slowly add the milk in small amounts at a time until you use it all.
Add the double cream and stir to combine (optional).
Add the grated cheddar, reserving some aside. Season with salt and pepper and stir until the cheese is melted in.
Remove the sauce from the heat and add in the mozzarella and stir until just melted.
Once your pasta is cooked, drain it. Then combine the pasta with the cooked bacon and cheese sauce. Mix until combined.
Spread evenly into an ovenproof dish and top with the reserved cheddar cheese.
Bake in the oven for 15-20 minutes or until the cheese on top is a golden brown colour.
Remove from the oven and allow it to sit for 1 minute. Serve and then serve yourself some more because it's really good - there will be no leftovers.