Preheat your oven to 350°F (175°C). Grease and flour a 9-inch cake pan.
In a large bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Mix until well combined.
Stir in the boiling water until the batter is smooth. Pour the batter into the prepared cake pan.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
To make the ganache, heat the heavy cream in a saucepan over medium heat until it begins to simmer. Remove from heat and add the chopped chocolate. Let sit for 5 minutes, then stir until smooth.
Pour the ganache over the cooled cake, allowing it to drip down the sides. Let the ganache set before serving.