Preheat your oven to 350°F (175°C).
Grease your 9x13-inch baking pan with a little vegetable oil or butter, and optionally line the bottom with parchment paper for easy removal.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, Dutch-processed cocoa powder, and salt until well combined.
In a double boiler or microwave-safe bowl, melt the finely chopped bittersweet chocolate until smooth.
In a separate bowl, combine the whole milk, vegetable oil, eggs, and vanilla extract. Whisk until well blended. Stir in the melted chocolate until fully incorporated.
Pour the wet ingredients into the bowl with the dry ingredients. Gently fold the mixture together using a spatula until just combined.
Pour the batter into your prepared baking pan and spread it evenly. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and place it on a cooling rack. Allow it to cool completely in the pan before frosting.
In a saucepan, heat the heavy cream over medium heat until it just begins to simmer. Pour the hot cream over the finely chopped milk chocolate in a bowl. Let it sit for a minute, then stir until the chocolate is completely melted and the mixture is smooth.
Add the softened unsalted butter to the ganache and stir until fully combined.
Once the cake is completely cool, pour the ganache over the top of the cake, using a spatula to spread it evenly. Allow the ganache to set before slicing.
