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, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed with an electric mixer until well combined, about 2 minutes.
Carefully stir in the boiling water. The batter will be thin; this is normal.
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 allow them to cool in the pans for 10 minutes.
Turn the cakes out onto wire racks and cool completely before frosting.
To make the chocolate ganache, place the chopped semisweet chocolate in a 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 3 minutes to melt.
Stir gently until the mixture is smooth and glossy. Allow the ganache to cool at room temperature until it thickens to a spreadable consistency, about 30 minutes.
Place one cake layer on a serving plate. Spread a generous layer of ganache 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 swirls.
For the ganache design, use the back of a spoon or a small offset spatula to create wave patterns or swoops around the cake.
Allow the ganache to set at room temperature for 1 hour before serving.