1. Preheat the oven to 350º/180°C.
2. Butter or spray a bundt cake pan and coat it with fine dry breadcrumbs.

For the sour cream ripple:

1. Mix sour cream, sugar, egg, rum (or milk), and vanilla in a small bowl. Reserve.

For the cake:

1. In a medium bowl, mix cocoa powder with water. Reserve.
2. In a large bowl beat butter and gradually add sugar. Beat until very creamy, about 3 minutes.
3. Add eggs, one at a time, mixing well after each addition. Beat for 1 minute after the last one is added.
4. Add vanilla and sour cream to the chocolate mixture and mix.
5. Sift flour, baking soda, powder, and salt.
6. Add dry ingredients in three additions to the butter mixture, alternating with the chocolate mixture in two additions. Don't worry if the batter looks a bit curdled at times.
7. Pour into prepared pan, smooth top. Make an indentation with a spoon in the middle of the batter. It will not be too defined, don't worry.
8. Mix pecans into sour cream ripple mixture. Pour carefully into the indentation, trying not to let it touch the sides.
9. Bake for 60 minutes or until a tester comes out clean.
10. Let cool a few minutes and unmold on a wire rack. Let cool completely.
11. Pour glaze slowly over cake and sprinkle top with chopped pecans.

For the glaze:

1. Mix sour cream, sugar, and vanilla in a small bowl until smooth.