Melt the dark chocolate and oil together in short intervals, stirring after each, until smooth.

Whisk the eggs and sugar in a large bowl until well combined. Add the melted chocolate mixture and whisk until smooth.

Sift in the cocoa powder. Add the self-rising flour and water and stir just until the batter is smooth.

Pour the batter into the prepared pan. Bake until a toothpick inserted in the center comes out with a few moist crumbs, 30 to 35 minutes, depending on the size of the pan.

Let the cake cool completely in the pan.

Make the ganache: Combine the dark chocolate, milk chocolate and heavy cream in a small saucepan over medium heat. Cook, stirring occasionally, just until the chocolate has melted and the mixture is smooth. Do not let it boil.

Pour the ganache over the cooled cake. Refrigerate until the ganache is set, about 1 hour. Slice and serve.