Special equipment: 12 lollipop sticks or decorative straws
For the cake pops: Line a baking sheet with parchment paper. Combine the cake crumbs and frosting in a large bowl. Work the frosting into the crumbs with your hands until it forms a thick paste and can be easily rolled into balls.
Roll into 12 balls and place on the prepared baking sheet using a 1-ounce ice-cream scoop. Freeze until almost solid, 20 to 30 minutes.
For the decoration: Melt the black chocolate discs in the top of a double boiler or in the microwave. Put the sanding sugar into a bowl. Remove the cake pop balls from the freezer and insert a lollipop stick or straw into each. Dip a cake ball into the black chocolate and swirl to coat completely, allowing any excess to drip off. Sprinkle generously with sanding sugar. Stand the cake pop, stick up, on the prepared baking sheet. Dip the ends of 8 licorice lengths into the black chocolate and attach to the cake pop to form the spider's legs. Repeat with the remaining cake balls.
Use a toothpick to dab a tiny bit of black chocolate to adhere the candy eyes. Let set completely at room temperature or in the refrigerator until ready to serve.
In the bowl of a stand mixer fitted with the whisk attachment, combine the confectioners' sugar, butter, milk, vanilla if using, and salt, and whip on low speed until incorporated. Turn the speed up to high and whip until the buttercream is smooth, scraping down the sides of the bowl once or twice. If it's too wet, add a bit more confectioners' sugar. If it's too stiff, add a bit more butter.
Use immediately or store in an airtight container at room temperature for 24 hours or in the fridge for up to 2 weeks. Rewhip cold buttercream before using it.