Lightly grease a 9x13-inch baking pan.
In a large saucepan over low heat, melt the butter.
Add the marshmallows, and stir until the marshmallows are melted, and the mixture is smooth.
Add the Cocoa Pebbles, and stir until the cereal is thoroughly coated.
Press the mixture evenly into the baking pan.
In a medium saucepan over low heat, melt the chocolate chips and shortening until smooth.
Stir in the mint extract.
Spread the melted chocolate over the marshmallow treats.
Sprinkle with mint chocolate baking chips.
Allow to set, at least 1 hour.
Cut into squares, and DEVOUR!