1. Prep Pan

   Preheat oven to 400°F. Grease a 15×10-inch baking pan. Cook macaroni according to package directions; drain.

2. Cook Bacon

   Meanwhile, in a 4- to 5-qt. pot cook bacon over medium until crisp. Drain on paper towels, reserving drippings in pot.

3. Make Cheese Sauce

   Add 2 Tbsp. of the butter to pot with bacon drippings; melt over medium. Add flour; cook, whisking constantly, until flour is light brown and smells toasty, about 2 minutes. Gradually whisk in milk. Whisk in next five ingredients (through pepper). Remove from heat; gradually whisk in 21/2 cups of the Gruyère and 1/4 cup of the Parmigiano Reggiano until smooth. Whisk in pumpkin. Add pasta and 1 cup of the Gruyère; stir until combined. 

4. Bake

   Spread mixture in prepared baking pan. Sprinkle with remaining 1/2 cup Gruyère. Bake until cheese is bubbly, about 10 minutes. Remove from oven. Preheat broiler

5. Add Topping and Broil

   In a bowl microwave remaining 2 Tbsp. butter until melted. Stir in panko and remaining 1/4 cup Parmigiano Reggiano. Sprinkle over macaroni. Broil 4 inches from heat until panko is toasted, 1 to 2 minutes. Sprinkle with bacon and chives.

How to Store Sheet Pan Mac and Cheese

Transfer leftovers to an airtight container. Chill in the fridge for up to 1 day.

How to Make Pumpkin Mac and Cheese Ahead

Prepare as directed through Step 3. Spread mixture in prepared pan. Cover and chill up to 24 hours. Bring to room temperature before baking. Bake as directed, except increase baking time to 15 minutes. Top and broil as directed.