Cookies: Preheat oven to 350° F, line cookie sheets with parchment paper.
In a large bowl cream butter and sugar; add eggs and vanilla, beat until fluffy. In a separate bowl combine dry ingredients. Add to creamed mixture and stir well. Fold in chocolate chips and walnuts, if using. Shape cookies into balls by using a rounded tablespoonful at a time and place at least 2 1/2 inches apart on prepared cookie sheets.
Bake at 350° F for 10 to 12 minutes; cool on cookie sheets before making spiders.
Frosting: In a medium bowl whisk together the powdered sugar and cocoa. In a separate bowl, beat the butter until smooth and add the sugar/cocoa mixture, 1/2 cup at a time, beating until incorporated. Add 2 tablespoons of the heavy cream, vanilla extract and beat well on medium speed. If the mixture is too stiff, add more milk until the mixture is firm and creamy.
To assemble spiders, put about a tablespoon of frosting on the flat side of one cookie. Make a sandwich by placing the flat side of another cookie on top. Make eyes with decorating gel, place on a plate and poke three stick pretzels on each side of spider to make legs.
Makes 12 - 18 spiders, depending on size of cookies.