Step 1Heat the oven to 375 degrees. Grease 2 baking sheets, then sprinkle lightly with water (to release cookies from pan after they are baked); set aside.
Step 2Cream the butter and sugar in the bowl of a mixer until fluffy, about 1 minute. Add the egg, vanilla and syrup. Mix until smooth. Add the cocoa, flour and salt. Mix well. Add the walnuts and oatmeal. Mix until combined.
Step 3Drop the batter by 1/2-tablespoon measures, about 2 inches apart, on the prepared pans. Use a finger dipped in water to flatten each cookie. Bake until the cookies are darker around the edges, about 7 to 8 minutes. Let them rest on the pan until firm enough to transfer (with a metal spatula) to a cooling rack. Cool completely. The cookies can be kept up to 1 week, stored in an airtight container at room temperature.