Line an 8x8" (20x20cm) pan with parchment paper or foil (if with foil, grease lightly with oil), and set aside.
Heat-treat the flour in one of two ways. First, place it in a microwave-safe dish and heat it in 10 second increments until it's reached above 160°F (71°C). Make sure to stir in between each increment. You'll have to use an instant read thermometer for this. A simpler way is to scatter the flour on a baking tray and baking it at 350°F for 7 minutes. Allow to cool completely.
1 cup flour
In a medium-sized saucepan, melt butter, white chocolate chips, sweetened condensed milk, and brown sugar. Stir on low. Sometimes white chocolate takes a long time to melt, so make sure it's completely smooth!
1/3 cup butter, 2 cups white chocolate chips, 14 oz sweetened condensed milk, 1/3 cup brown sugar
Add in flour and stir.
1 cup flour
Once combined, add in vanilla and continue mixing.
1 tbsp vanilla
Working quickly, add in the semi-sweet chocolate chips. Only stir a few times. Since the fudge is warm, the chocolate will begin to melt immediately. Do not over-mix or else it will turn into a chocolate fudge! We want to see beautiful streaks.
3/4 cup semi-sweet chocolate chips
Immediately pour into prepared baking pan and spread evenly. Top with extra chocolate chips (optional) and place into the fridge to set. This typically takes 2-3 hours.
Once set, slice and enjoy!