Preheat oven to 350 degrees. Line the bottom of an 8-inch square baking pan with parchment. Whisk to combine flour, cocoa powder, salt, and cinnamon in a bowl.

In a separate bowl, beat brown sugar, molasses, and butter with an electric mixer on medium speed until smooth, 1 to 2 minutes; beat in eggs. Reduce speed to low; add flour mixture, beating just until combined. Fold in orange peel, dates, and walnuts.

Transfer batter to pan; smooth top with an offset spatula. Bake until surface is set and a toothpick inserted in center comes out with only a few moist crumbs clinging to it, 20 to 25 minutes. Let cake rest in pan 5 minutes. Invert onto a wire rack; peel off parchment. Brush bottom side (now facing up) with 3 tablespoons port. Let stand 30 minutes. Flip cake over on rack, and brush other side with remaining tablespoon port. Let stand 1 hour. (Cake can be kept at room temperature, wrapped well in plastic, up to 1 month.)

Cut into 3/4-inch squares, wiping knife with a damp cloth between cuts. Squares can be refrigerated in an airtight container (layered between sheets of waxed paper) up to 1 week.