Preheat oven to 350 degrees. In large bowl, cream butter and both sugars until well blended. Add eggs and vanilla extract, and mix to combine.
In separate bowl, sift flour, cocoa powder, baking soda and salt. Slowly add dry ingredients to butter mixture, and stir until combined.
Fold in candies and walnuts, if desired. Chill dough 1 hour, or overnight. Drop dough by rounded tablespoons onto lightly greased tray, about 2 inches apart.
Bake for 7-9 minutes for chewy cookies, or 10-12 minutes for crispy cookies.
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