Prepare your pan by lining an 8×8 inch baking dish with parchment paper, leaving overhang on the sides for easy removal later. This prevents sticking and makes clean-up a breeze.
Heat-treat the flour by spreading it on a microwave-safe plate and microwaving in 30-second intervals, stirring between each, until it reaches 165°F (about 1-2 minutes total). Let cool completely before using.
Make the cookie dough layer by creaming ½ cup of softened butter with brown sugar and granulated sugar in a large bowl until light and fluffy, about 2-3 minutes. Add vanilla extract and mix well.
Add the dry ingredients by mixing in the cooled heat-treated flour and ½ teaspoon salt until just combined. Stir in milk until the mixture resembles cookie dough consistency. Fold in ½ cup of mini chocolate chips.
Press the cookie dough evenly into the bottom of your prepared pan, creating a smooth layer. Place in the refrigerator while preparing the fudge layer.
Create the fudge layer by combining the remaining ½ cup butter, sweetened condensed milk, white chocolate chips, and remaining ½ teaspoon salt in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until smooth and fully melted.
Pour the fudge mixture over the chilled cookie dough layer, spreading it evenly with a spatula. Work quickly as it will begin to set.
Sprinkle the remaining chocolate chips over the top of the warm fudge layer, pressing them in gently so they stick but remain visible.
Refrigerate the fudge for at least 2 hours, or until completely set and firm to the touch.
Cut into squares by lifting the fudge out of the pan using the parchment paper overhang, then slicing with a sharp knife. For cleaner cuts, wipe the knife between slices.