Preheat your oven to 180C and grease a 20cm cake tin.
Beat the butter and sugar together the add the eggs one a time while mixing until combined.
Fold in the flour, milk and vanilla extract until the mixture is smooth.
Divide the mixture between two bowls.
Stir the cocoa powder into one of the bowls until combined.
Take 2 spoons and use them to dollop the chocolate and vanilla mixtures into the cake tin one at a time.
When all both mixtures are sitting in the cake tin, use a skewer to gently create swirl patterns in the batter to create a marbled effect.
Bake the cake for 45-55 minutes or until a skewer inserted into the centre comes out clean.
Once cooked, remove from the oven and let cool on a wire rack.