01 - Preheat oven to 400°F. Lightly grease a 9x13 inch baking dish.
02 - Boil elbow macaroni in a large pot of salted water until al dente. Drain and set aside.
03 - In a medium saucepan, melt unsalted butter over medium heat. Whisk in flour and cook for 1 minute.
04 - Gradually whisk in whole milk until smooth. Stir in Dijon mustard, garlic powder, black pepper, and salt. Continue to stir until the sauce thickens, about 4 to 5 minutes.
05 - Remove saucepan from heat and stir in grated cheddar and Gruyère cheese. Mix until melted and the sauce is smooth.
06 - Add drained macaroni to the cheese sauce and mix thoroughly. Transfer the mixture to the prepared baking dish and spread evenly.
07 - In a mixing bowl, whisk together flour, baking powder, and salt. Cut in cold cubed butter using a pastry cutter or fork until the mixture resembles coarse crumbs.
08 - Stir in milk and chopped chives until the dough is just combined. If the dough appears dry, add a small splash of extra milk.
09 - Drop spoonfuls of biscuit dough evenly over the macaroni and cheese layer.
10 - Place baking dish in oven and bake for 30 to 35 minutes, until biscuits are golden brown and baked through and the macaroni mixture is bubbling.
11 - Allow the casserole to stand for 5 minutes before serving.