Add 1 cup milk, 1 stick unsalted butter, and 1 teaspoon kosher salt to the water. Carefully add the Brussels sprouts. Reduce the heat to medium and boil, stirring occasionally, until the Brussels sprouts are tender, 10 to 12 minutes. Drain the sprouts (you can save the butter bath for other veggies), then return to the pot. Add 2 tablespoons Dijon mustard and 2 teaspoons maple syrup and toss to combine.