Special equipment: 24 lollipop sticks, 4 pastry bags, 3 number 2 decorating tips, 1 number 1 decorating tip and a foam block
For the green velvet cake: Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking dish with nonstick cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and granulated sugar until light and fluffy. In a small bowl, whisk together the eggs and vanilla until well combined. Add half of the eggs to the butter mixture, mix until completely incorporated and then add in the remaining eggs and mix again until completely incorporated.
In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt, then set aside. In another bowl, whisk together the sour cream and milk until smooth.
Starting and ending with the dry ingredients, alternate adding the flour and the milk mixtures to the stand mixer. Mix until combined, scraping down the sides and bottom of the bowl after each wet addition. Use a rubber spatula to fold in the gel color. Pour the batter into the prepared pan and bake until a toothpick inserted in the center comes out clean, 35 to 45 minutes. Cool the cake completely before using.
For the buttercream frosting: In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy. Add the confectioners' sugar and mix on low speed until all the sugar is mixed in. Add the milk, vanilla and salt and mix on low speed until the liquid is absorbed, then increase the speed to medium high and beat until light and fluffy. Place in a medium bowl and cover with plastic wrap until ready to use.
For the royal icing: In the bowl of a stand mixer fitted with the whisk attachment, whisk the egg whites and vanilla until frothy. Add the confectioners' sugar 1/4 cup at a time until smooth and mixed thoroughly. Add the lemon juice and mix until combined.
For the cauldron cake pops: Using your hands, crumble the green velvet cake into a large bowl. Add 1 cup of the buttercream frosting and mix until evenly combined. Line a baking sheet with parchment paper. Roll the cake mixture into 1 1/2-inch balls and place on the prepared baking sheet. Freeze the cake balls until set, about 30 minutes.
Melt about 5 ounces of the black candy melts. Dip the ends of the lollipop sticks in the melted candy and push the sticks halfway into the cake balls (the melted candy will act as an adhesive). Place the cake pops in the refrigerator to chill for about 30 minutes.
Melt the remaining 7 ounces black candy melts. Dip the cake pops into the melted candy to cover completely, allowing the excess candy to drip back into the bowl. Immediately place the cake pops on the lined baking sheet.
This creates a flat surface on the top of the cake pop, which will be the rim of the cauldron.
Divide the royal icing among 4 bowls. Tint each bowl with a gel color: black, green, orange and red. Scoop each frosting into its own pastry bag fitted with a tip; orange gets a number 1 tip and the black, green and red get number 2 tips. Pipe the red icing onto the bottom of each cauldron, starting from the lollipop stick towards the center, to look like flames. Pipe the orange icing over the red icing for more detail.
Flip the pops upright and put the lollipop sticks in the foam block to secure them. Use the black icing to pipe 2 small half circles on opposite sides of the rim of each cauldron to create the handles. Pipe the green icing on top of the cake pops for the magic potion. Place some green candies on top to look like bubbles. You can use tweezers for better placement control. Let the pops set and then serve.
Use the royal icing immediately or store in an airtight container at room temperature for 3 to 5 days.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.