Place heavy cream in a saucepan over low-medium heat. Bring the heavy cream to a simmer, but not boiling. Remove the cream from the heat source as soon as it starts simmering.
Place your finely chopped chocolate in a heat proof bowl. Pour the hot cream over the chocolate and let it sit for a couple of minutes.
Mix the chocolate and cream together. Be patient. It won't look like it's coming together, but it will eventually. Keep mixing until you get a homogenous mixture.
Let the dark chocolate ganache cool down before you use it on your cake. Let it cool for 15-20 minutes.
Place your cake on a wire rack. The chocolate will flow off of it if you are encasing the cake in the ganache. Be prepared for spillage. You might want to place a plate underneath the wire rack to catch any drippings.
Pour the ganache over the cake. Make sure to go to the edges with the ganache to make it flow and flood evenly.
Let the ganache cool on the cake. You can place it in the fridge to speed up the process. The ganache should set after about an hour.
Serve the dark chocolate ganache cake within 1-2 days. Store in the refrigerator up for 3 days.