Preheat oven to 350° and grease an 8- by 8-inch or 9- by 9-inch square metal baking pan.
Sift together flour, baking soda, baking powder, salt, cinnamon and set aside. Using an electric mixer, cream butter and brown sugar until light and fluffy. Mix in beaten egg and coffee. Then add the dry mixture to the wet coffee mixture. Stir in chocolate and nuts and pour into the prepared pan. Bake for 20 to 25 minutes, until set but still a little wobbly in the center.
Just after removing the cake from the oven, mix the remaining 2 tablespoons of coffee with the confectioner’s sugar until smooth. While still warm, ice cake with the mixture by pouring it over the top. Let the icing firm slightly as the squares cool.
Adapted from Mrs. Harold C. Dodge, Block Island Cookbook: A Collection of Favorite Recipes.