Step #1 Put the stock into a saucepan & warm through.
Step #2 Slake the cornflour - stir the tspful into a little of the liquid - then add it to the saucepan & stir it in.
Step #3 Chop the butter into half inch cubes & add to the pan.
Step #4 Whisk it over a medium heat, melting the butter & thickening the sauce slightly until it is the consistency of single cream & a lovely pale gold, Spice up to taste.
Step #5 NB: Chefs don't add the cornflour, that is, that you don't boil it! If you do, but its addition ensures that the beurre blanc doesn't separate, provided, it will become oily & separate even with the cornflour.