Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until well combined.
Carefully stir in the boiling water until the batter is smooth. The batter will be thin.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and let them cool in the pans for 10 minutes. Then turn them out onto wire racks to cool completely.
To prepare the chocolate ganache, place the chopped semisweet chocolate in a medium heatproof bowl.
Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Do not boil.
Pour the hot cream over the chopped chocolate and let it sit for 2 minutes to soften.
Add the butter to the chocolate and cream mixture, then gently stir until the ganache is smooth and glossy.
Allow the ganache to cool at room temperature until it thickens to a spreadable consistency, about 20-30 minutes.
Place one cake layer on a serving plate and spread a layer of ganache evenly over the top.
Place the second cake layer on top and cover the entire cake with the remaining ganache using a spatula to create smooth or decorative patterns as desired.
For a design, use the back of a spoon or a spatula to create swirls or waves in the ganache before it sets.
Allow the ganache to set at room temperature for about 1 hour before slicing and serving.