In a saucepan, add sugar and water. Heat on medium flame. Cook until sugar dissolves. Turn the heat off and allow it to cool down.
Take a medium-sized bowl, pour puffed rice, m&m’s, sugar sprinkles and sugar syrup. Mix well. Pour the mixture in the baking pan lined with parchment paper. Using a spatula or hand press into the pan but be gentle.
Keep it in the freezer for 1 hour. Demould the rice krispie cake. Pipe the buttercream frosting on it. Scatter m&m’s and sprinkles over the top. Serve.