Place cubed sweet potatoes in a large pot and cover with cold water by 2 inches. Bring to a boil over high heat, then reduce to medium and cook until completely tender when pierced with a fork, about 12-15 minutes. Drain and return to the pot.

Add warm milk to the cooked sweet potatoes and blend with an immersion blender (or transfer to a stand blender) until completely smooth and velvety, like thick soup. Set aside.

Bring a large pot of salted water to a rolling boil. Add pasta and cook 2 minutes less than package directions for al dente. Drain and set aside. The pasta will finish cooking in the oven.

In the same large pot used for sweet potatoes, melt 3 tablespoons butter over medium heat. Whisk in flour and cook, stirring constantly, until the mixture turns light golden and smells nutty, about 2 minutes. Do not let it brown.

Slowly whisk in the sweet potato puree until smooth. The mixture will thicken quickly. Reduce heat to low and add cheddar and gruyere by handfuls, stirring until each addition melts completely. Stir in Dijon mustard and cayenne if using. Taste and adjust salt as needed.

Add drained pasta to the cheese sauce and stir until every piece is coated. The sauce should look slightly loose; it will tighten as it bakes. Transfer to a greased 9x13 inch baking dish.

Toss panko with melted butter and parmesan, then sprinkle evenly over the top. Bake at 375°F until bubbling at the edges and the topping is golden brown, about 20-25 minutes. Let rest 10 minutes before serving so the sauce sets.