1. Lightly coat a 15-by-10-inch rimmed baking sheet with cooking spray.
2. Place cereal in a large bowl coated with cooking spray; set aside. Melt butter in a medium saucepan over medium heat and add marshmallows. Stir until smooth. Pour over cereal and stir to combine. Spread out in prepared pan and press firmly to flatten. Let stand until cool, about 1 hour. Trim edges and cut into 30 3-by-1-1/4-inch bars. Push a popsicle stick through one narrow end of each bar.
3. Using an offset spatula, spread a thin layer of white chocolate over tops of 15 bars. Repeat, using semisweet chocolate for remaining 15 bars. Transfer to a baking sheet and refrigerate until set, about 20 minutes. Place a wire rack over another baking sheet. Dip the white bars in white chocolate and the semisweet bars in semisweet chocolate; place on rack, allowing excess chocolate to drip off bars. Refrigerate until firm, about 20 minutes. Makes 30 pops.To paint faces on bars::
4. Add 1 or 2 drops of food coloring into remaining white chocolate and stir to combine. Using the tip of a wooden skewer or a paintbrush, dip in pink chocolate, then paint ears and nose on bars. Use semisweet chocolate and white chocolate to make eyes. Refrigerate until firm, about 30 minutes. (Can be made ahead. Store in an airtight container, in a single layer, in the refrigerator up to 2 days. Serve cold or at room temperature.)