Prepare a baking sheet by lining with parchment paper, silicon mat, or rubbing on butter. Set aside.

Add butter to a large pot over medium heat. When melted, add marshmallows to the pot and simmer, stirring occasionally, until fully melted.

Remove the pot from the heat and add Rice Krispies cereal. Stir gently with a spatula until fully combined. Keep in mind that as it cools, the mixture will get harder to work with.

Pour onto the prepared baking sheet. Press and shape with your hands until smooth and even. Cool completely (about 1 hour).

Once completely cooled, remove the block of Rice Krispies treats from the pan and place it on a cutting board. Using a spider web cookie cutter, cut as many as you can get.

Using a gel frosting tube, draw a web on the cookies. Then place a spider sprinkle on each one.