01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish.
02 - Cook macaroni in a large pot of salted boiling water until al dente. Drain and set aside.
03 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute until mixture is bubbling.
04 - Gradually whisk in milk until smooth. Stir in Dijon mustard, garlic powder, pepper, and salt. Cook, stirring frequently, until the sauce thickens, about 4–5 minutes.
05 - Remove the pan from heat and stir in grated cheddar and Gruyère cheese. Mix until the cheese is fully melted and sauce is smooth.
06 - Add drained macaroni to the cheese sauce and mix to coat evenly. Transfer macaroni mixture to prepared baking dish, spreading it out in an even layer.
07 - In a mixing bowl, whisk together flour, baking powder, and salt. Cut in cold cubed butter using a pastry cutter or fork until mixture forms coarse crumbs.
08 - Stir in milk and chopped chives just until combined. Do not overmix. If dough seems dry, add a splash of extra milk.
09 - Drop spoonfuls of biscuit dough evenly across the macaroni and cheese.
10 - Bake for 30–35 minutes, or until biscuits are golden brown and cooked through, and filling is bubbling.
11 - Allow to stand for 5 minutes before serving to set the layers.