Step 1

In a heat proof bowl, combine egg yolks, sugar, key lime zest, key lime juice and salt. Whisk to combine.

Step 2

Place bowl over a double boiler and cook over medium-low heat, whisking constantly until thickened. It takes about 5-7 minutes.

Step 3

Remove from heat and add butter. Stir until smooth. Place in a jar. Let it cool to room temperature, then refrigerate. Keep in the fridge for up to 1 week.