Line an 8x8 square pan with aluminum foil and lightly butter the sides and bottom of the foil. Set aside.
Combine condensed milk, white chocolate chips, butter, and brown sugar in a medium-sized saucepan over medium-low heat. Stir frequently until chocolate and butter are melted and ingredients are well-combined.
Reduce heat to low. Add flour and corn starch, stir well until completely combined and mixture is smooth.
Add vanilla and salt, stir well.
Pour about ⅓-1/2 of the fudge into prepared pan. Immediately sprinkle about half of your chocolate chips over the top of the batter, then pour remaining fudge over top.
Sprinkle remaining chocolate chips over the top of the fudge. Use a knife to swirl slightly.
Allow to cool completely before cutting and serving -- several hours at room temperature or about 1 hour in refrigerator.