Preheat oven to 375°F. In a large mixing bowl, cream butter and sugars at medium high speed until light and fluffy. Add eggs, one at a time, beating well after each addition. Add vanilla. Sift together flour, cocoa, baking soda and salt. Add flour mixture gradually and beat well. Shape dough into small log and place in cookie press. Press cookie onto cool ungreased cookie sheets. Bake at 375°F for 10-12 minutes. Remove from sheet and cool. Store in airtight container at cool room temperature for several weeks or freeze for two months.