Melt the unsweetened and semi-sweet chocolate.
Ways To Melt Chocolate:
Melt chocolate in the top of a double boiler over hot water (do not let the boiling water touch the bottom of the top pan). Once melted, remove from heat.
In a small heavy-bottom saucepan over low heat melt the chocolates, stirring often to prevent burning. Remove from heat and cool.
Place chocolate into a microwave-safe glass bowl; heat until soft at 50% power approximately 2 minutes. Remove from microwave and stir chocolate until smooth.
In a medium bowl, combine egg yolks and sugar. Stir in heavy cream; add to the melted chocolate. Cook, stirring constantly, until thickened. Remove from heat and let cool.
In a medium bowl, cream butter until light and fluffy; beat into chocolate mixture, 1 tablespoon at a time.
Refrigerate until frosting is of spreading consistency.
Chocolate Frosting Recipe: https://whatscookingamerica.net/Cake/ChocolateFrosting.htm