Lightly spray an 8 x 8-inch baking pan with nonstick cooking spray and line the pan with parchment paper, leaving a 1-2 inch overhang on either side. Set aside.
Add 3 cups high-quality white chocolate chips, 1 (14-ounce) can sweetened condensed milk, and 2 tablespoons salted butter to a medium saucepan and heat over medium-low heat, stirring constantly until fully melted and very smooth, about 4 minutes.
Remove from the heat and stir 4 tablespoons of the 7 tablespoons cinnamon sugar into the fudge mixture.
Pour the fudge mixture into the prepared baking pan and smooth over the top. Spoon the remaining 3 tablespoons of cinnamon sugar evenly over the top, then swirl it into the fudge with a knife.
Cover the pan with aluminum foil or plastic wrap and chill in the refrigerator for at least 2 hours, or until solid.