Heat oven to 350°F. Grease and lightly flour two 9-inch round layer-cake pans.
Prepare both boxes of cake mix together as directed on package. Pour half into each prepared pan.
Bake 35 to 40 minutes until a pick inserted near center comes out clean.
Cool in pans on rack 10 minutes. Invert cakes on rack and cool completely.
Place one cake layer on serving plate. Tuck strips of waxed paper under edges of cake (to keep plate free of frosting). Remove 1/4 cup frosting from can to a cup; reserve for spider-web design. Scrape remaining frosting into a bowl. Stir in orange food color until frosting is lighter than desired shade (color will darken on standing). Spread cake layer with 1/3 cup, cover with second cake layer and frosting sides and top with remaining frosting. Pull out waxed paper.
To make spiderweb: Stir black food color into reserved frosting. Scrape into a sandwich-size plastic bag. Snip off a corner and squeeze icing in a web design on top and sides of cake. Pipe a spider or two on web.
Cookies: Heat oven to 350°F. Lightly grease cookie sheet(s). Have cookie cutters (see Note) or homemade heavy-cardboard patterns ready.
Beat butter and sugar in a large bowl with electric mixer until pale and fluffy. Beat in egg, milk and vanilla until blended. With mixer on low speed gradually beat in the flour just until blended.
Divide dough in thirds. Pat each piece into a 1-inch-thick rectangle. Wrap separately and refrigerate 1 hour or until firm enough to roll.
On lightly floured surface with lightly floured rolling pin, roll one piece of dough at a time (keep the rest refrigerated) to 1/4-inch thickness. Cut in desired shapes with cookie cutters or patterns. Place wooden craft sticks on prepared cookie sheet(s). Place cut-out dough on top and press down slightly (allow at least 1 inch between cookies).
If using cookie cutters to make ghosts: Before you bake, lengthen ghosts' "sheets" to look as if they're trailing by shaping dough with your fingers. Reroll scraps and chill.
Bake 8 to 10 minutes or just until bottom and edges of cookies are golden brown. Cool on cookie sheets 2 minutes before removing to wire rack to cool completely. Repeat with remaining dough.
To decorate iced cookies: For each color, put a small portion of frosting into a cup. Add paste color, a little at a time, until frosting is lighter than desired shade (color will darken on standing). Stir in water, a few drops at a time, until frosting is thin enough to apply with a brush or icing spatula, but thick enough to coat without dripping.
Spread icing to edge of cookies. Before icing sets, add licorice-lace hair, bat and pumpkin decorations and sprinkle pumpkin with orange-colored sugar. Let icing dry at least 2 hours before adding piped-on features (to pipe, see Step 6 in Halloween Cake recipe).
To decorate white ghosts: Pipe on features. Coat with a thick layer of confectioners' sugar stirred through a fine strainer. Remove excess sugar on features by dabbing with a wet brush. Let piped icing dry before sticking cookies into cake or placing on platter. Makes about sixty 3-inch cookies.
