In a large bowl, beat the butter, sugar, sour cream, egg, and vanilla on medium speed of electric mixer until smooth. Gradually beat in the flour, cocoa, baking soda and salt on low speed until all the ingredients are mixed. Divide the dough into two flattened rounds. Wrap each in plastic wrap and refrigerate for 1 hour. Heat the oven to 350 degrees. On a lightly floured board, roll the dough to 1/4-inch thickness. Cut with 3-inch gingerbread cutter. Place 2-inches apart on ungreased cookie sheets. Bake for 8 to 10 minutes or until the edges are set. Cool on the cookie sheet for 2 minutes. Remove the cookies to a cooling rack. Cool completely (about 20 minutes). Decorate as desired.