Preheat oven to 350°F (175°C). Cook macaroni according to package directions. Drain and set aside.
In a large saucepan, melt butter over medium heat. Whisk in flour until smooth. Gradually whisk in milk and Guinness until smooth. Bring to a simmer, stirring constantly.
Reduce heat to low and stir in cheddar cheese until melted and smooth. Season with salt and pepper.
Add cooked macaroni to the cheese sauce and stir to coat. Pour into a greased 9x13 inch baking dish.
Bake for 20-25 minutes, or until golden brown and bubbly.
Notes
Optional: Top with breadcrumbs before baking for extra crunch.