Preheat oven to 350°F. Grease and flour two 9-inch round cake pans. In a large bowl, combine cake mix, eggs, oil, and water. Mix on medium speed for 2 minutes. Divide batter evenly between pans.

Bake the cakes for 25-30 minutes or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes, then remove from pans to cool completely on a wire rack.

Heat heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and pour over the chopped chocolate. Let sit for 5 minutes, then stir until smooth and glossy.

Allow the ganache to cool slightly before using. It should be thick but still pourable for spreading between cake layers and on top.

In a mixing bowl, beat the butter until creamy. Gradually add powdered sugar and cocoa powder, alternating with milk, beating until smooth and spreadable.

Place one cake layer on a serving platter. Spread half of the ganache over the cake layer. Top with the second cake layer and spread remaining ganache on top. Let set before frosting.

Use a spatula to frost the sides and top of the cake with the prepared frosting. Smooth out the edges for a clean finish or create decorative swirls.

Slice the cake into 8 pieces and serve with a scoop of vanilla ice cream or a dollop of whipped cream for an extra indulgent treat.

Store the cake in an airtight container at room temperature for up to 3 days, or refrigerate for up to 5 days.