Preheat oven to 350°F (170°C). Using a mixer with the paddle attachment, beat the butter, margarine, cocoa powder, brown sugar and granulated sugar until creamy. Add vanilla. Add eggs, one at a time, beating well after each addition. Beat in half of the flour, the baking soda and the salt. Beat in the remaining flour. Stir in chocolate chips and nuts (if using). The dough will be very stiff.
Drop dough mounds of about 1 inch (2.5 cm) in diameter onto baking sheet lined with parchment paper. Bake at 350°F (170°C) for 9-11 minutes, just until the surface of the cookies loses its shine. Cool on baking sheet for 2 minutes then carefully loosen cookies from the parchment paper and move them to wire cooling racks.
Cookies will stay fresh in an airtight container at room temperature for up to one week or in the freezer for up to one month.
Based on chocolate chip cookie recipe from Better Homes and Gardens New Cookbook.
The Perfect Chocolate Chocolate Chip Cookies https://livingoncookies.com/the-perfect-chocolate-chocolate-chip-cookies/