Preheat oven to 180C. Grease a bundt cake pan with butter and dust it with cocoa powder, this is optional but I have this kind of paranoia of my cakes sticking to the tin because mine's made of aluminum so I wasn't sure how non-stick it would be even with butter.
Whisk the dry ingredients together in a bowl; the flour, salt, baking powder.
Mix the cocoa powder with the milk and sugar in a jug.
Beat the butter with sugar till creamy and soft. Add eggs one at a time until well mixed. Add vanilla extract.
Gradually add the dry ingredients and the 1 cup milk alternatively.
Take out about 2 and 1/2 cups of batter and pour in the chocolate mix from the jug. Stir or whisk till well incorporated.
Use a spoon or pour the batter into piping bags and create layers. First pour the normal batter into the base. Pour the chocolate into the middle and around it. And pour the normal mix in the middle of the chocolate layer again. Repeat till you've used up all the batter.
Bake for 50-60 mins till a cake tester comes out clean with inserted into the mix.
Let it cool in the pan for 20-30 mins before removing it onto a rack.
Dust it with icing sugar or make a glaze and pour it over the top.