Preheat the oven to 350?F (175?C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, whisk together the self-rising flour, sugar, and cocoa powder until well combined.

Add the milk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix until smooth.

Carefully stir in the boiling water until fully incorporated. The batter will be thin.

Pour the batter into the prepared cake pan.

Bake in the preheated oven for 30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.