Preheat your oven to 350°F (180°C) and grease a 9-inch round cake pan.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.
In a separate bowl, whisk together the water, vegetable oil, vinegar, eggs and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until well combined.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake the cake for 30-35 minutes, or until a toothpick inserted into the centre comes out clean.
Let the cake cool completely before frosting.
In a medium-sized mixing bowl, beat the butter until smooth.
Gradually add the powdered sugar, cocoa powder, milk, and vanilla extract, beating until smooth and creamy.
Spread the frosting over the cooled cake, making sure to cover the top and sides completely.
Serve the cake and enjoy!