Preheat the oven to 350°. Butter and flour a 10-cup Bundt pan. In a large bowl, sift the flour with the cocoa powder, baking soda and salt. In a standing electric mixer fitted with the paddle attachment, beat the softened butter until creamy. Add the granulated sugar and beat at medium speed until light and fluffy. Add the eggs, 1 at a time, beating well after each addition. Beat in the vanilla extract. At low speed, beat in the dry ingredients in 3 additions, alternating with 3 additions of the sour cream. Gradually beat in the boiling water.