Grease baking sheet; set aside. Chop 6 KIT KAT bars. Melt butter in large saucepan set over low heat; stir in marshmallows for about 6 minutes or until completely melted. Stir in vanilla. Remove from heat.
Immediately stir in cereal and chopped KIT KAT bars until coated. Scrape onto prepared baking sheet; using wet hands or buttered spatula, spread evenly.
Let cool slightly; transfer to cutting board and trim edges. Cut into 3 long strips. Cut each strip into 12 triangles. Cut a small part from top of each triangle to create a flat edge; make an indent in the top with a chopstick or teaspoon handle.
Spoon milk chocolate into resealable bag; snip bottom corner. Break remaining KIT KAT bars in half along seam. Pipe some chocolate into indents of triangles and affix 1 KIT KAT bar into each, pressing down lightly into rice cereal treat.
Spoon white chocolate into resealable bag; snip bottom corner. Pipe or drizzle remaining milk chocolate and white chocolate over rice cereal treats to resemble bristles. Let stand for about 1 hour or until chocolate is set.