In a medium heatproof bowl, combine the egg yolks and sugar. Set the bowl over a saucepan of barely simmering water and stir until the yolk mixture is hot to the touch. Remove the bowl from the heat and, using an electric mixer, beat the yolks until doubled in volume. Add the chocolate and beat on low speed just until incorporated; the mixture will stiffen. Beat in the melted butter. Add 2 tablespoons of the heavy cream and beat on medium speed until smooth.
In another bowl, whip the remaining heavy cream until stiff peaks form. Beat one-quarter of the whipped cream into the warm chocolate mixture, then fold in the remaining whipped cream. Pour the chocolate cream into a serving bowl and refrigerate until firm, preferably overnight.
The chocolate cream can be refrigerated for up to 3 days.