Heat the oven to 350°F and grease a 1-quart oven-safe bowl.
In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the eggs, milk, oil, and vanilla extract. Stir until the batter is smooth and no dry streaks remain.
Pour the batter into the prepared bowl and bake for 35 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cake in the bowl for 10 minutes, then turn it out onto a wire rack to cool completely.
Once cool, spread the chocolate ganache evenly over the dome to create a smooth black hat surface.
Place each dome on a round chocolate cookie to form the brim of the witch hat before serving.