Not really sure how to title this one.
If you want a quick easy thin layer of chocolate in your cake, pop half your vanilla cake batter into your tin, sprinkle on some drinking chocolate, Milo or even Nesquik (which has many flavours) then spoon on the other half of your cake batter. Bake your cake as per instructions.
I must say don’t do too thick a layer, there is nothing worse than trying to turn out your cake to have it split in half as the top and bottom have not adhered to each other, so if you can try to make specific points of the batter joining. When cut this is a vanilla/butter cake with a marbled chocolate line roughly through the middle.
You can also do this with some grated milk or dark chocolate, just a little more work but same effect.
Try just dropping a little Nesquik in the middle of a cupcake.
I like to do this with a circle cake tin (so you have a round cake with the hole in the middle) and drizzle chocolate icing roughly over the top and sides.
I’ve also heard that you can use jelly crystals instead of the chocolate, this gives great colour but I personally have not tried this.