Place the finely chopped semisweet chocolate in a heatproof bowl.
In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Do not let it boil.
Pour the hot cream over the chopped chocolate and let it sit undisturbed for 2 minutes to soften the chocolate.
Gently stir the mixture with a whisk or spatula until the chocolate is completely melted and the ganache is smooth and glossy.
Add the unsalted butter to the ganache and stir until fully incorporated for extra shine and smoothness.
Allow the ganache to cool at room temperature until it thickens slightly but is still pourable, about 10 minutes.
Pour or spread the ganache over your cooled chocolate cake using a spatula or spoon to decorate and glaze the cake evenly.
Let the ganache set at room temperature for about 30 minutes before serving.