Combine unsalted butter, heavy cream, and dark chocolate in a saucepan over low to medium heat, stirring frequently.

Sift in the cocoa powder, powdered sugar, and kosher salt until fully combined and glossy, about 5-7 minutes.

Stir gently for an additional 2-3 minutes until smooth and silky.

Remove from heat and strain the mixture into a bowl using a fine-mesh sieve.

Cover the surface with plastic wrap to prevent a skin from forming.

Let it cool at room temperature for about 30 minutes, then refrigerate for 1-2 hours.

Before use, stir well to bring back the creamy consistency; gently reheat if too thick.