Beat the eggs and sugar together until pale and frothy.
Add the oil and milk, and beat again until well combined.
Sift the flour, cocoa powder, and baking powder together in a separate bowl, then fold gently into the batter.
Pour the batter into a greased 27 cm baking pan.
Bake at 180°C (350°F) for about 30 minutes, or until a toothpick inserted in the center comes out clean.
While the cake bakes, combine the sauce ingredients in a saucepan and bring to a boil.
Reserve 1.5 cups of the sauce, then stir in the canned milk and dark chocolate until melted and smooth.
Let the baked cake cool for 15–20 minutes, then slice it.
Slowly drizzle the plain sauce over the cake, a little at a time, to avoid making it mushy.
Once absorbed, pour the reserved chocolate mixture over the top, saving a little for serving.
Let the cake rest for 30 minutes before serving.