Sift together the flour, cocoa powder, and salt into a small bowl. Set aside. In a liquid measuring cup combine baking soda and buttermilk. Set aside.
Using an electric mixer, beat butter, sugar, eggs, and vanilla until light and fluffy, about 30 seconds. Stir in sour cream on low speed. Add the flour mixture and buttermilk mixture alternatively, starting and ending with flour mixture. Beat batter for 2 minutes on medium speed. Pour batter evenly into prepared pan.
Bake for 40-50 minutes, or until a wooden toothpick inserted into the center comes out clean. Cool in pan for 10 minutes then remove cake to a wire rack to cool completely. Glaze with orange glaze (recipe below).
In a medium bowl combine all ingredients and beat until smooth.