Use the leftover egg yolks from the Eton mess recipe to make the rich buttery hollandaise sauce that makes this breakfast dish so special. Put the butter in a small pan and heat slowly until it has melted and the fat solids separate and sink to the bottom. Remove from the heat and pour the liquid butter into a small bowl and set aside. Discard the solids. Bring a pan of salted water to a gentle simmer. Put the egg yolks in a large mixing bowl and sit the bowl over the water, making sure the bottom of the bowl doesn't touch the water. Add the vinegar and some seasoning and whisk the eggs for a minute or so. Slowly pour the melted butter into the egg yolk mixture, whisking continually until all the butter has been incorporated and you have a sauce of spooning consistency. Add a little hot water, if necessary. Set aside. Heat the grill to high and grill the bacon for a few minutes until crisp, turning hallway through. Drain on kitchen paper and keep warm. Poach the eggs in the pan of water for 4 minutes, then scoop out and drain. Slice the bagels in half and toast in a toaster for a couple of minutes. Top each toasted bagel half with a couple of rashers of bacon, a soft poached egg and a generous spoonful of the hollandaise sauce. Garnish with the chives and serve immediately.