In a medium saucepan, combine butter, heavy cream, dark chocolate, cocoa powder, powdered sugar, and salt. Heat over medium-low, whisking constantly until the mixture is completely melted and glossy.
The mixture should look like a smooth, liquid Ganache with no visible sugar grains.
Pour the mixture through a fine-mesh sieve into a large bowl or shallow baking pan.
Seal and Cool Press plastic wrap directly onto the surface of the frosting to prevent a skin from forming. Fridge: Transfer to the refrigerator for 1–2 hours. Room Temp: Let it sit for 30 minutes to stabilize.
Remove from the fridge once it reaches your desired thickness. Give it a quick stir with a sturdy spatula to restore its creaminess.
Notes
The "Shallow Pan" Hack: To cool the frosting 50% faster, pour it into a wide, shallow glass baking dish instead of a deep bowl. More surface area means a faster, more even set.
The Sieve is Non-Negotiable: Even if your cocoa looks fine, sifting it directly into the saucepan is the only way to guarantee a "silk-sheet" finish.
Avoid Overheating: Never let the mixture reach a rolling boil. High heat can "break" the chocolate emulsion, leading to a greasy texture rather than a fudgy one.
Temperature Matters: For the perfect "Matilda Cake" look, use the frosting when it is slightly cooler than room temperature (about 18°C/64°F). It will be thick enough to hold its shape but soft enough to swirl effortlessly.
Yield: This recipe makes enough to cover one 8 or 9-inch cake, a Bundt cake, or 12 cupcakes. If you are making a double or triple-layer cake, or 24 cupcakes, just double the recipe.