Cook macaroni according to package instructions. Drain and set aside.
Melt butter in medium saucepan over medium heat. In bowl, whisk arrowroot or cornstarch into heavy cream until thoroughly combined, then slowly add cream mixture to butter in pan. Bring to simmer, stirring frequently, until mixture thickens. Reduce heat to low. Stir in cheese, mustard powder, salt and pepper. Add cooked macaroni and stir until thoroughly coated.
Refrigerate macaroni and cheese until well chilled, at least 1 hour. Once chilled, stir in beaten egg.
Spread 1 cup almond flour in shallow dish and line 2 baking sheets with parchment or waxed paper. Using clean hands, form macaroni into balls about 2 inches in diameter, squeezing gently to press them into shape. Roll carefully in almond flour and place on prepared baking sheets.
Place baking sheets in the freezer until macaroni and cheese balls are firm, about 30 minutes. Preheat oven to 400°F and bake 12 to 15 minutes. Turn oven to broil and place pan under broiler for 4 minutes, until almond flour begins to brown and crisp.