Using a heavy knife, finely chop the semi-sweet chocolate bars into uniform, pea-sized pieces. Place them into a heatproof glass mixing bowl.

Pour the heavy whipping cream into a saucepan over medium heat. Bring it just to a gentle simmer with tiny bubbles at the edges. Remove immediately from the heat.

Pour the hot cream over the chopped chocolate, ensuring all pieces are submerged. Let the bowl sit completely undisturbed for exactly 5 minutes.

Using a whisk, begin stirring the mixture in the center in slow, tight circles. Once a dark, glossy core forms, slowly widen your circles until the mixture is fully smooth and cohesive.

Stir in the room-temperature unsalted butter, vanilla extract, and kosher salt until completely melted and absorbed, resulting in a deep brown sheen.

Allow the ganache to sit at room temperature for 60-90 minutes until it reaches a peanut-butter consistency. Scoop onto your dark crumb chocolate sponge cake and use a silver offset spatula to spread a thick, one-inch layer with textured, swirling ridges.