Prepare a sous vide water bath (set to 175F)

Bring heavy cream to a simmer. Whisk in dark chocolate baking chips, erythritol, vanilla extract, and salt with the cream. Remove from heat.

In a separate pot, bring bone broth to a boil, reducing while working with the heavy cream. Once reduced, whisk into cream mixture a little at a time.

Whisk in egg yolk one at a time until mixture is smooth. Transfer to pourable measuring cup to easily divide mixture.

Divide into 4-oz mason jars. Screw on lids fingertip tight. Place sealed jars in the water bath.

Cook custards in the water bath for 1 hour. Remove using tongs and place the jars in ice and water to cool.

Set for 2.5 hours, then serve with whipped cream and sprinkle with cacao powder or shaved dark chocolate.