Line an 8x8 baking dish with parchment or foil, making sure to have it hanging over the edges (for easier fudge removal). Coat the parchment or foil lightly with cooking spray.
In a small saucepan, over low heat, melt the white chocolate chips.
In a separate saucepan, melt the milk chocolate chips over low heat.
Add half of the sweetened condensed milk to the melted white chocolate chips and the other half to the melted milk chocolate chips.
Remove the pans from the heat and stir them until fully melted and well combined.
Add the mint extract and the green food coloring to the white chocolate chips, and stir until fully combined.
Working quickly, drop a few spoonfuls of each one into the foil-lined baking pan. Continue adding randomly until both saucepans are empty.
Shake the 8x8 pan a little and tap it firmly on the counter to remove air bubbles.
Very gently run a butter knife or wooden skewer through the batter a few times to give it a swirled effect.
Place into the refrigerator for at least two hours.
Remove from the refrigerator and pull the hanging edges of the foil (with the fudge) out of the pan and away from the fudge.
Cut into small square pieces–a little goes a long way! Enjoy.
Store in the refrigerator in an airtight container.