Bake brownies according to directions to the box of brownies. Let cool completely.
Once cool, add powdered sugar to a small bowl and add the cream and vanilla extract. Whisk to mix until the frosting is smooth.
Cut brownies into 16 brownies. Add frosting to a small ziploc bag, putting the frosting into the corner of the bag. Cut the edge of the bag off, making a very small hole. To make the spiderweb, start a spiral design from the middle of the brownie, moving outwards as you go. Then use a toothpick to drag from the center of the spiral, repeating multiple times to create the spiderweb! Place on a spooky platter and top with fake spiders!