Preheat your oven to 350°F (175°C). Grease and flour the two 9-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes.
Carefully stir in the boiling water until well combined. The batter will be thin.
Pour an equal amount of batter into each prepared pan. Bake for 30 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, transfer the cakes to a cooling rack to cool completely.
For the ganache, heat the heavy cream in a double boiler or microwave until just simmering (do not boil).
Pour the hot cream over the chopped chocolate in a mixing bowl. Let it sit for 5 minutes, then add the butter and stir until smooth and glossy.
Place one layer of the cooled cake on a serving platter. Spread a layer of ganache on top. Add the second cake layer and cover the top and sides with the remaining ganache.
Let the ganache set for about 30 minutes before slicing and serving.