Preheat your oven to 350F. (regular bake setting/not fan assisted). Grease a 10-inch tube (angel food) cake pan and set aside. *Alternately, you can use a 10-inch bundt pan and serve it flipped over.
Whisk together the flour, baking powder and salt in a medium bowl. Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer fitted with a paddle attachment, beat together the sugar and oil. Add the eggs, one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Stir in the vanilla until blended. Add the flour mixer alternately with the milk, mixing just until combined.
Measure out 2 cups of the batter and add to a medium bowl. Stir in the melted chocolate and the chopped chocolate until well combined.
Spoon 1/2 of the plain batter to your prepared pan, covering the bottom of the pan. Spoon the chocolate batter in a ring on top, keeping it in the centre and leaving the inner and outer edges clear. Top with the remaining plain batter.
Bake in preheated oven for 50-55 minutes, or until a tester inserted in the centre comes out clean. Cool in pan for 15 minutes, then run a knife around the outside and the inner ring. Remove cake to a cooling rack to cool completely.