Break up or chop chocolate into small pieces and set aside with butter.

Add egg yolks, sugar, water, and salt to a metal mixing bowl. Cook, whisking, directly over medium-low heat until the mixture is thick, foamy, and hot to the touch (145 to 150 degrees F).

Once the yolk mixture is thick and hot, add chocolate and butter, and whisk until all chocolate is melted. Let rest for a few minutes, whisking occasionally to cool to room temperature.

Whisk cold cream until medium stiff peaks form. Avoid over-whipping to prevent a grainy texture.

Transfer 1/3 of the chocolate mixture into whipped cream and fold with a spatula. Fold remaining chocolate gently to retain air.