Preheat the oven to 400 degrees F.
Bring a large pot of water to a boil and add the cauliflower florets.
Cook the florets for 4 minutes and then drain in a colander. Set aside.
In a large pan melt the 3 tbsp of butter and then add the flour.
Whisk together over a medium/low heat and gradually add the milk whisking all of the time.
After 2 cups of milk have been added, your sauce should look smooth with no lumps.
Add 2 cups of the grated Cabot Cheddar and whisk until melted.
Add the remaining half cup of milk iif the sauce is too thick for you after adding the cheese.
Add the rosemary, salt and pepper and whisk together.
Add the cauliflower and chopped bacon to a large bowl and then pour the cheese sauce on top of it.
Gently stir the sauce through the cauliflower and bacon until well coated then transfer everything to your prepared baking dish.
Mix the melted butter and panko breadcrumbs together then sprinkle over the surface of the cauliflower followed by the remaining 1/2 cup grated cheddar.
Place on a baking sheet pan and bake in the oven for 25 minutes.
After 25 minutes switch to your broiler and broil for 2-3 minutes and the surface is nice and golden.
Let the cauliflower mac and cheese sit for 5 minutes before serving.