Pop the popcorn according to package instructions. Remove any unpopped kernels and place the popcorn in a large mixing bowl.
In a saucepan over medium heat, melt the butter and add marshmallows. Stir continuously until melted and smooth.
Remove from heat and stir in mint extract and green food coloring, if using.
Pour the marshmallow mixture over the popcorn and mix gently to coat.
Let cool for 2 minutes, then fold in chocolate chips and mint pieces.
Press the mixture into a greased 9x13-inch pan using wax paper or buttered hands.
Let the bars set completely at room temperature, then cut into squares and serve.