Using a standing mixer, beat the egg whites, powdered sugar, vanilla, and cream of tartar for 5-7 minutes until stiff peaks form. You made need to add 1⁄2 Cup more of powdered sugar.
Divide the icing into 4 bowls. Mix 1 color into each bowl until you get the desired colors you like.
Cut 12 circles out of the brownie bar and place them onto a cookie sheet.
Pipe a triangle and other odd shapes onto the brownie and fill the sections in, making sure you color the brownie with each of the colors.
Repeat the same step with the remaining colors, making sure they touch
Using the black icing bag, pipe black lines over the parts where the colors meet to create Sally's stitching. Pipe small lines or x over the black lines to create the binding look of sally's dress Allow the icing to harden.
Squeeze the black frosting into the last piping bag. Pipe a spider anywhere on the top of the brownies.
Allow the frosting to harden for an hour before enjoying.
Notes
Sam's Club or Costco carry pans of brownies or you can make a box mix.