In a large saucepan, over low heat, melt the butter.
Add the marshmallows and cook and stir until they melt. Remove from the heat.
Add chocolate chips and vanilla and stir until the chips are melted and fully incorporated.
Stir in cereal until fully coated.
Turn into a lightly buttered 9 x13" pan.
Press the mixture into the pan to make an even layer. Immediately add your sprinkles and press gently down on them with the palms of you hands to help them adhere.
You can leave at room temperature for a few hours (or you can chill in the refrigerator). Then cut into 24 square (4 by 6 cuts). Cut each square into 2 triangles.
Store, covered, at room temperature. They won't last very long!