Whisk all ingredients except the butter in a saucepan on low heat. Keep whisking/stirring constantly with a whisk or wooden spoon for about 5-9 minutes – it varies based on how low your ‘low heat’ is and also based on how big your eggs were. You will know when it is done – it suddenly becomes thick – keep on stirring/whisking and remove from heat. Add the butter slices and stir until smooth. Pour into a glass mason jar if not using right away and store in the fridge.