Preheat oven to 350 degrees F (177 degrees C) and place rack in center of oven.  Butter, or spray with a non stick spray, a 10 inch (25 cm) bundt or tube pan.  

In a stainless steel bowl, over a saucepan of simmering water, melt the semisweet chocolate with the coffee.  Remove from heat and set aside.

In a separate bowl sift together the flour, baking powder, baking soda, salt, and ground cinnamon.  Set aside.

In bowl of electric mixer, cream the butter.  Gradually add the sugar and continue to beat until the mixture is light and fluffy (about 5 minutes).  Add beaten eggs, one at a time, mixing well after each addition.  Beat in the vanilla extract and sour cream.

With the mixer on low speed alternately add the flour mixture and the milk to the batter, in three additions, beginning and ending with the flour.  

After preparing the batter, pour half of the batter into a separate bowl.  Stir the melted chocolate into one half of the batter, mixing well.  Place the batter into the prepared pan by alternating spoons of vanilla batter and chocolate batter.  Then, with the end of a wooden spoon, gently draw swirls through the batter to marbleize it.  Don't over mix or you won't have that wonderful marble affect.

Bake for about one hour, or until a toothpick inserted in the center comes out clean.  Remove from oven and place on a wire rack to cool for about 10 minutes before removing the cake from the pan to cool completely.  Serve warm or at room temperature.  This cake will keep for a couple of days at room temperature or it can be frozen.

Adapted from:  Yard, Sherry. 'The Secrets of Baking'. Houghton Mifflin Company. New York: 2003.