Preheat oven to 350F. Grease three 8-inch cake pans, and line bottoms with parchment paper.
In a large bowl, mix the flour, sugar, baking powder, baking soda and salt.
In a bowl of standing mixer with a paddle attachment, mix buttermilk, egg, oil, vanilla and vinegar on medium speed until well combined.
Reduce the speed to low and slowly add the dry ingredients. Mix until just incorporated.
Evenly distribute the batter into the prepared cake pans and spread evenly.
Bake for 18 to 20 minutes or until wooden pick inserted in cake comes out clean. Cool in pan on wire rack for about 10 minutes before inverting on rack. Let cool completely before frosting.
To prepare the frosting, beat butter for a few minutes with a mixer with the paddle attachment on medium speed. Add powdered sugar and turn your mixer on the lowest speed until the sugar has incorporated with the butter. Increase mixer speed to medium and add vanilla extract and cream. Then beat for another 3 minutes. Add color, one drop at a time, and continue mixing and adding color until it reaches the preferred color.
To assemble the cake, place one cake layer on your serving plate and spread with 1/3 cup frosting. Place another cake layer on top, and gently press down a little bit. Spread with another 1/3 cup frosting. Place the last layer of cake on top, smear a thin layer of frosting on the top and sides of the cake.
Use an open star tip, close star tip or hair tip to pipe the rest frosting on cake. Starting from the bottom, and move up to the top of the cake.
To prepare the monster face, melt white and black candy melts in two separate microwavable piping bags.
Use the white candy melts to pipe small triangles on a parchment paper for teeths.
Use the black candy melts to pipe a long and thick line on a parchment paper for mouth.
For the cake pops, pipe a solid white circle on each of them. Let it dry, then pipe a black dot on.
Cover and refrigerate the cake. Remove from refrigerator an hour before serving. Place cake pop eyes, mouth and teeth on top of the cake.