Preheat oven to 350F. Butter a 13x9-inch cake pan.
Melt ½ cup of butter. In a large bowl, whisk together cake mix, 1¼ cups milk, melted butter, and eggs until combined. Pour into the cake pan.
Bake according to package directions. Cool completely.
In a large mixing bowl or stand mixer fitted with the paddle attachment, beat the remaining 1½ cups of butter until smooth. Slowly beat in the confectioners' sugar. Beat in the milk. Increase the speed to medium-low, and beat for 1 minute.
Remove all but 1 cup frosting from the bowl. Add the cocoa and 1 tablespoon milk. Beat until well combined. Crumble the cooled cake into the chocolate frosting.
Beat on low speed until the mixture resembles wet sand. Mix in sprinkles. Scoop and roll into 1-inch balls. Place on a wax or parchment paper-lined baking sheet and freeze for 20 minutes.
Melt the candy or chocolate according to package directions. Dip the tip of a lollipop stick in melted candy and insert it halfway into a cake ball. Dip the cake ball into melted candy to coat, tapping on the sides of the bowl to remove excess. Insert the end of the stick into a foam block. Repeat with the remaining cake balls. (Only take a few cake balls out of the freezer at a time so that they stay cold and the coating sets quickly.)
Cut fruit leather to make mouths; use melted candy to paint on fangs. Attach mouths and candy eyes to cake balls by dabbing melted candy on the back of each one.
When set, divide reserved frosting and color as desired. Place in piping bags with grass piping tips. Pipe onto cake balls to create fur (have fun!). Add antennas or other decorations as desired.