Preheat the oven to 375 degrees and have ready a 8-10 cup oven-safe casserole dish.
Cook the pasta as per the manufacturers instructions. Do not drain.
Once the pasta has cooked, scoop it out of the pot with a slotted spoon and transfer to a large bowl leaving the pasta water in the pot.
Add 1-2 cups of water to the pot and bring back to a boil. Add the broccoli florets and blanch for two minutes. Drain and set the broccoli to the side.
Cook the chopped bacon in a pan until crispy, Transfer to a paper towel and drain.
In another pan, melt the butter over a medium heat and addd the flour. Whisk to mix together.
Gradually add the milk about 1/4 cup at a time whisking well after each addition until smooth.
Once all of the milk except about 1/2 cup has been added add the cheese and whisk gently over a low heat until melted and lump free.
Add the remaining milk and mix. Turn to low heat.
Add the broccoli and bacon to the cooked pasta along with the cheese sauce.
Toss with a wooden spoon making sure to cover all of the pasta. Pour the pasta into the casserole dish.
In a smaller pan melt the remaining 4 tbsp butter and add the Panko, pepper and rosemary. Mix well and sprinkle over the macaroni.
Place in the oven and bake for about 35-40 minutes and the cheese is bubbling around the edges.
Serve at once.