Whisk all ingredients, except for the butter, in a temperature-safe bowl (such as Pyrex) to combine. Add the butter to the mixture.
Place the bowl in the microwave and run for 30 seconds on full power. Remove the bowl and whisk the contents thoroughly. Repeat this process 10 times or until the mixture has thickened and the temperature reads 170ยบ fahrenheit using an instant thermometer.
Place the bowl into an ice bath to cool completely. Store in an airtight container in the refrigerator for up to 7 days.