Step 1: Cook the Rotini - Bring a large pot of well salted water to a boil, then add the rotini and cook it according to the package directions until al dente. You want the pasta tender but still holding its shape, since it will spend another minute or two in the warm sauce. Before draining, scoop out about half a cup of pasta water and set it aside. You may not need it, but it is useful if the sauce needs loosening later. Drain the pasta well and leave it nearby while you start the skillet.
Step 2: Build the Garlic Butter Base - Set a large skillet or Dutch oven over medium heat and melt the butter. Add the minced garlic and stir for about 30 seconds, just until it smells fragrant. Keep an eye on it because garlic can go from golden to bitter quickly. Sprinkle in the flour and whisk constantly for one full minute. This brief cooking time gets rid of the raw flour taste and creates the thickening base that will help the sauce wrap around the pasta instead of sliding to the bottom of the bowl.
Step 3: Melt the Creamy White Cheddar Sauce - Slowly pour in the milk while whisking the whole time. Adding it gradually helps prevent lumps, so do not rush this part. Once the mixture looks smooth, stir in the softened cream cheese, shredded white cheddar, onion powder, salt, and black pepper. Lower the heat if the sauce starts bubbling hard. Gentle heat is kinder to cheese and helps it melt into a smooth sauce. Keep stirring until the cheeses are completely melted and the sauce looks glossy, creamy, and thick enough to coat the back of a spoon.
Step 4: Fold In the Chicken and Cranberries - Add the drained rotini and diced cooked chicken to the skillet first, then stir until both are coated in the sauce and warmed through. Fold in the dried cranberries and chopped pecans, making sure they are spread throughout instead of gathering in one corner. If the sauce feels thicker than you like, add a splash of milk or a little reserved pasta water and stir again. Remove the skillet from the heat, scatter the chopped chives over the top, and serve while the sauce is still soft and creamy.