Line an 8x8 or 9x9 pan with parchment leaving overhang. Spray with cooking spray. Set aside.
In a double boiler over simmering water, melt together semi sweet chocolate chips with 2 Tbsp butter. Do not let water touch bottom of bowl. Once melted add ½ sweetened condensed milk. Mix until fully combined, then add marshmallow cream.
Meanwhile, in a microwave safe bowl using the melt function or on 50% power, melt white chocolate with 2 Tbsp butter. Melt in 30 second increments, stopping to stir each time. Repeat until smooth. Remove from microwave, mix in remaining sweetened condensed milk and mint extract. Add a few drops green food coloring until desired color is reached.
Once semi sweet chocolate is melted add ½ sweetened condensed milk. Mix until fully combined, then add marshmallow cream. Mix until marshmallow cream has melted.
Pour chocolate into pan. Top with white chocolate. use a knife or a skewer to swirl. Cool to room temperature then chill for 4 hours or just until set.
Remove fudge from pan using parchment. Cut into 36 one inch pieces.
Store at room temperature in an airtight container for up to 2 weeks.
This recipe also works well using low fat (not fat free) sweetened condensed milk.