Preheat the oven to 350°F. Prepare a 3-inch deep 6-inch cake pan Wilton pan from Amazon) by brushing the bottom and sides with melted butter and dusting it with cocoa, then lining the bottom with a round of parchment. Set aside.
Stir together the vinegar and the milk in a cup measurer and let it stand for a few minutes.
Sift together the flour, baking soda, salt, and cocoa into a large bowl. Whisk in the sugar.
Add the oil, the vanilla, and the soured milk to the bowl with the dry ingredients. Stir (or whisk) to combine, just like cake mix.
Pour the batter in the prepared pan and bake it until a cake tester inserted in the center comes out clean (about 45 minutes).
Let the cake cool for 10 minutes before unmolding it onto a cooling rack to cool completely.
While the cake is cooling, prepare the ganache by heating the cream til it is hot and steamy, then pour it over the chocolate in a medium metal bowl.
Let the mixture stand for one minute, then stir slowly with a wooden spoon until the chocolate is melted and the ganache is fluid and smooth.
Let the ganache cool slightly to thicken while you set the wire rack with cooled cake over a rimmed baking sheet.
When the ganache has thickened slightly, pour it over the cooled cake. Transfer the cake to a cake stand to serve.