Preheat the oven to 180°C. Grease and line a 23 cm round cake tin with parchment paper.
In a large bowl, sift together 200 grams all-purpose flour, 75 grams cocoa powder, 1 1/2 teaspoons baking powder, and 1 1/2 teaspoons baking soda. Set aside.
In another bowl, whisk together 200 grams granulated sugar, 2 large eggs, 120 ml vegetable oil, and 1 teaspoon vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, alternating with 240 ml buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Stir in 240 ml boiling water until the batter is smooth. The batter will be thin, but this is normal.
Pour the batter into the prepared cake tin and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
For the frosting, melt 200 grams dark chocolate and allow it to cool slightly. In a separate bowl, beat 200 grams unsalted butter until creamy. Gradually add 250 grams powdered sugar, 1/2 teaspoon vanilla extract, and the melted chocolate, mixing until smooth and fluffy.
Once the cake is completely cooled, spread the frosting evenly over the top and sides of the cake.