Melt semi-sweet chocolate chips (½ cup) in the microwave or in a small saucepan on low heat, stirring frequently. Once melted, remove from heat, stir until smooth, and then set aside. Allow to cool at least 5 minutes before proceeding.
In a separate large bowl, beat the softened unsalted butter (½ cup, i.e., 1 stick) on medium speed using a hand or stand mixer (with paddle attachment) for 2-3 minutes (until creamy).
Slowly add in powdered sugar (1 ½ cups), followed by the melted chocolate, heavy cream(¼ cup), vanilla extract (1 teaspoon), and mint extract (½ teaspoon), and continue mixing on high speed for 2 minutes. Add crushed thin mint cookies. Once everything is added, slowly turn the mixer up to high speed and beat for 3-4 minutes.
Notes
If frosting is too thin, feel free to add more powdered sugar until it is your desired consistency. If frosting is too thick, you may add more heavy cream.