Preheat the oven to 350 degrees F and adjust the rack to the center position. Bring a large pot of water to boil. Add the pasta shell and cooke until al dente, according to the package direction. Drain the pasta in a colander. 

Return the pot to the stove over medium-low heat. Melt the butter in the pot and then add the milk, arrowroot, sea salt and pepper. Whisk the sauce until throughly combined and then add the pasta back to the pot. Top with half of the cheese (12 ounces). Stir until the cheese is melted and the pasta is thickly coated with the creamy sauce. 

Pour the pasta into a large baking dish. Top with the remaining cheese and bake until the cheese is melted and bubbly and just turning golden brown on the top, about 25 minutes. Serve.