01 - Preheat oven to 400°F. Lightly grease a 9x13 inch baking dish.
02 - Boil the elbow macaroni in a large pot of salted water until al dente. Drain and set aside.
03 - Melt 2 tablespoons butter in a saucepan over medium heat. Stir in 2 tablespoons flour; cook for 1 minute.
04 - Gradually whisk in 2 cups whole milk until smooth. Add Dijon mustard, garlic powder, black pepper, and salt. Cook, stirring, until the sauce thickens, about 4 to 5 minutes.
05 - Remove saucepan from heat. Add grated cheddar and Gruyère or mozzarella, stirring until melted and smooth.
06 - Fold the drained macaroni into the cheese sauce. Transfer to the prepared baking dish and spread evenly.
07 - In a bowl, whisk together 1 1/2 cups flour, baking powder, and salt. Cut in cold cubed butter using a pastry cutter or fork until mixture resembles coarse crumbs.
08 - Stir in 1/2 cup milk and chopped chives just until a soft dough forms; add extra milk if needed. Do not overmix.
09 - Drop spoonfuls of biscuit dough over the macaroni and cheese filling.
10 - Bake for 30 to 35 minutes until biscuits are golden and cooked through and the filling is bubbling.
11 - Allow to stand for 5 minutes before serving.