Lightly spray an 8x8 or 9x9 pan with cooking spray and line it with parchment paper and set it aside.
Melt together the white chocolate chips and sweetened condensed milk in a microwave-safe bowl. You can do this in a double boiler, stirring until melted or microwaving for 30 seconds at a time, stirring in between, until melted.
Once melted, sift in the matcha powder and mix until no clumps remain. Then, pour the mixture into your prepared pan.
If a dark chocolate swirl is desired, melt the dark chocolate in the same bowl as you mixed the matcha and white chocolate. You do not need to clean it because the dark chocolate will not turn green. Once melted, drizzle the dark chocolate over the matcha fudge using your rubber spatula. Then draw a skewer along parallel lines in the fudge, approximately 1-inch apart.
Transfer to the refrigerator for 1 hour until set. Then, cut the fudge into 1-inch squares.
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Notes
The fudge does not need to be refrigerated. However, if it is very warm in the house, you might want to keep it refrigerated so it isn't too soft.Sweetened condensed milk is not the same as evaporated milk and evaporated milk does not work in this recipe.