Heat oven to 170°C fan bake. Line a 23cm square cake tin with nonstick baking paper. Place milk and vinegar in a large bowl and stir to make soured milk. Whisk in the sugar, egg, salt, oil and vanilla essence.
Sift together the flour, cocoa powder and baking soda and add to the wet mixture, stirring just enough to combine into a smooth batter.
Pour into the prepared cake tin and bake for 30 minutes or until the center springs back when pressed with a fingertip. Remove to cool, then ice with chocolate frosting.
Beat butter and icing sugar together in a bowl.
Add cocoa powder and boiling water and beat until creamy. Makes 1 cup.