Heat the stock in the small saucepan. Then in the other saucepan melt the butter and add the flour to it. Stir with a wooden spoon over a low heat for 2-3 minutes to create a blond roux.
Make the sauce - Gradually, add the hot stock into the pan with the flour and melted butter and stir constantly to blend the ingredients together. Bring to the boil, stirring all the time.If you stop stirring or the leave the sauce on the heat unattended, it will stick and burn.
Lower the heat and simmer until the sauce reaches the desired thickness, about 10-15 minutes.
Season the sauce with salt and a couple of turns of pepper and allow it to cool for a couple of minutes.
Finally, beat the egg yolks into the sauce one at a time until well incorporated.
Strain the sauce through the sieve into a bowl or another pan.