Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving overhang on two sides to lift the fudge out later. Lightly spray the parchment with nonstick spray if desired.
nonstick spray, parchment paper
Heat-treat the flour for safe edible cookie dough. Spread the flour on a baking sheet and bake at 300°F (150°C) for 5–7 minutes, stirring once, until it reaches 165°F, or microwave in a microwave-safe bowl in 30-second bursts, stirring between each, until it reaches 165°F. Let cool completely, then whisk to break up any lumps.
1 1/4 cups all-purpose flour
In a medium bowl, beat the softened butter, brown sugar, and granulated sugar with a hand mixer or stand mixer on medium speed for 2–3 minutes, until light, creamy, and slightly lighter in color.
1/2 cup unsalted butter, 1/2 cup light brown sugar, 1/4 cup granulated sugar
Add the milk, vanilla, and salt to the creamed mixture and mix until combined.
2 tablespoons milk, 1 teaspoon vanilla extract, 1/4 teaspoon fine sea salt
Add the cooled, heat-treated flour in two additions, mixing on low speed just until no streaks remain. The dough should be thick but soft, like classic cookie dough. If it is very crumbly, add an extra teaspoon of milk.
1 1/4 cups all-purpose flour, 2 tablespoons milk
Fold in the mini chocolate chips with a spatula so they don’t break up. Spread the dough evenly in the prepared pan, pressing it gently into a flat, even layer. Chill the pan while you make the fudge layer.
1/2 cup mini chocolate chips
In a medium saucepan, combine the white chocolate chips, sweetened condensed milk, and 2 tablespoons butter. Warm over low heat, stirring almost constantly, until the chocolate chips are just melted and the mixture is smooth. Do not let it simmer to avoid grainy fudge.
2 cups white chocolate chips, 14 oz can sweetened condensed milk, 2 tablespoons unsalted butter
Remove the pan from the heat and stir in the vanilla and a pinch of salt. The mixture should be thick, glossy, and pourable. If it looks slightly lumpy, keep stirring off the heat for another minute until it smooths out.
1 teaspoon vanilla extract, salt
Remove the chilled cookie dough layer from the fridge. Pour the warm fudge mixture evenly over the cookie dough and spread it gently into the corners. While still soft, sprinkle extra mini chocolate chips on top and lightly press them in so they stick.
1/2 cup mini chocolate chips
Refrigerate for at least 2–3 hours, or until the fudge is set and firm enough to cut. For very clean squares, chill overnight. Alternatively, chill for 1 hour, then transfer to the freezer for 20–30 minutes to finish firming.
Use the parchment overhang to lift the slab out of the pan onto a cutting board. Use a sharp knife to cut into 1–1 1/2-inch squares, wiping the blade with a warm, damp cloth between cuts for neat edges.