Directions

1. Make the cake: Preheat oven to 325 degrees. Butter a 14-cup Bundt pan. Whisk flour, cocoa, baking soda, and salt in a large bowl. Mix milk and sour cream in a small bowl.
2. Cream butter and sugar with a mixer until light and fluffy. Beat in eggs, 1 at a time, then vanilla. Reduce mixer speed to low, and add flour mixture, alternating with milk mixture, ending with flour. Fold in walnuts, if using. Spoon batter into pan. Bake until a tester inserted in center comes out clean, 55 to 60 minutes. Let cool in pan on a wire rack. Invert cake. (Cake will keep for up to 1 day.)
3. Make the glaze: Place chocolate in a bowl. Heat cream in a small saucepan until simmering, then pour over chocolate. Let stand for 2 minutes. Add butter and Cognac or rum, if using, and mix until smooth. Let stand, stirring occasionally, until slightly thickened. Pour glaze over cooled cake. Slice, and serve with tangerine segments.