Preheat your oven to 350°F (175°C).
Grease a 13x18-inch rimmed baking sheet with butter or non-stick spray. Optionally, line the bottom with parchment paper.
In a large mixing bowl, whisk together the flour, sugar, baking soda, salt, and cocoa powder until well combined.
Melt the bittersweet chocolate in a double boiler or microwave-safe bowl. Set aside to cool slightly.
In another bowl, whisk together the milk, vegetable oil, eggs, and vanilla extract. Stir in the melted chocolate until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined.
Pour the batter into the prepared baking sheet and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted comes out clean.
While the cake is cooling, heat the heavy cream in a saucepan until it simmers. Remove from heat and mix in the milk chocolate and butter until smooth.
Once the cake has cooled completely, spread the ganache evenly over the top.