For the yolk butter, mix the whipping cream with cornflour and bring to the boil while stirring with a whisk and reduce slightly. Beat or clarify the yolks in a bowl and whisk briskly into the hot mixture, then remove from the heat.
Transfer to a mixing bowl and whisk until cold. Gradually beat the softened butter into the cold-stirred mixture, as you would for a buttercream. Stir the finished, room-temperature yolk butter until very fluffy and stir in the flavorings, herbs and spices and season to taste.