This is the trickiest part. Start heating the water (to boiling). Meanwhile, stir the mixture with a whisk until it reaches a temperature of 140°F. This happens roughly when the water starts boiling.
It will start to look slightly more clear (see picture above).
Step 2: Remove from heat and transfer to a KitchenAid mixing bowl. Use mixer to whisk (on high setting) until stiff peaks form. Turn down the setting to "3" and continue whisking until the mixture is relatively cool to touch.
Step 3: Change from whisk to paddle and continue on setting "3" while adding chunks of butter, about a tablespoon at a time. Mix to emulsify.
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