Preheat oven to 350º. Grease a nonstick 12 cup bundt pan with cooking spray. In a large bowl, prepare the filling by mixing the brown sugar, cocoa, and cinnamon, walnuts and raisins.
In a large bowl, cream the butter, sugar, eggs and vanilla. Add the flour, baking soda, baking powder, and salt. Mix gently to combine. Fold in the sour cream.
Place 3 large scoops of the batter in the prepared bundt pan and spread it evenly. Sprinkle ½ cup of the filling over the batter. Place 3 large scoops of the batter over the filling and sprinkle the remaining filling over the batter. Finish with the remaining batter, spreading it evenly over the filling. Bake in oven for 40-45 minutes or until apick comes out clean. When done invert and place on a wire rack to cool.