Meanwhile, melt the butter in a medium saucepan over a medium heat. Once melted and foaming, stir in the flour to make a paste. Cook, stirring from time to time, for 2-3 mins, until slightly darkened. Mix in the garlic powder, mustard powder and cayenne pepper, if using, then gradually and slowly pour in the milk, splash by splash, whisking well after each addition. Pour in the beer and whisk it in. Bring to a simmer and leave to bubble for 5 mins until thickened and reduced slightly. Remove from the heat and whisk in the Worcestershire sauce and cheese. Pour into a warm dish and sprinkle with chives. Serve straightaway.