Spread 1 cup of all-purpose flour in a thin, even layer on a baking sheet. Bake at 300°F (150°C) for 5–8 minutes, stirring once, until the flour reaches at least 165°F (74°C). Let it cool completely, then whisk or sift to break up any clumps before measuring 1 cup (120 g) for the cookie dough layer.

Line an 8×8-inch (20×20 cm) baking pan with parchment paper, leaving some overhang as a handle. Lightly mist the parchment with nonstick spray if desired.

In a medium saucepan, combine the white chocolate chips, sweetened condensed milk, and 4 tablespoons butter. Place over low to medium-low heat and stir constantly until the chocolate is almost fully melted and the mixture is smooth and thick. Remove from heat and stir in 1 1/2 teaspoons vanilla and 1/4 teaspoon fine sea salt until glossy and completely smooth.

Immediately pour the fudge mixture into the prepared pan and spread into an even layer. Tap the pan gently on the counter to smooth the surface and release air bubbles. Let the fudge sit at room temperature for 15–20 minutes, until warm but not hot to the touch.

While the fudge cools, make the cookie dough. In a medium mixing bowl, beat together 1/2 cup softened butter, 1/2 cup packed light brown sugar, and 1/4 cup granulated sugar with a hand mixer on medium speed until light and creamy, about 2–3 minutes.

Add 1 teaspoon vanilla and 1/4 teaspoon fine sea salt to the creamed mixture and beat to combine. Mix in the cooled, heat-treated flour about half at a time on low speed until incorporated.

Add 2 tablespoons milk or heavy cream, 1 tablespoon at a time, mixing just until you have a soft, spreadable cookie dough that holds together. If needed, add a tiny splash more milk or a spoonful of flour to adjust. Fold in 1/2–3/4 cup mini chocolate chips with a spatula.

Drop spoonfuls of the cookie dough evenly over the slightly cooled fudge base. Using clean hands, a small offset spatula, or a piece of parchment, gently press and spread the dough into an even layer, trying not to press too deeply into the fudge.

If desired, sprinkle 2–3 tablespoons mini chocolate chips, sprinkles, and/or a pinch of flaky sea salt over the top and gently press so they adhere.

Cover the pan loosely with plastic wrap or foil and refrigerate for 2–3 hours, or until the fudge is completely firm. For best flavor and the cleanest slices, chill overnight.

Use the parchment overhang to lift the fudge out of the pan onto a cutting board. Warm a sharp knife under hot water, dry it, and cut the fudge into 1–1 1/2 inch squares, wiping the knife between cuts. Serve chilled or slightly softened.

Store squares in an airtight container in the refrigerator with parchment or wax paper between layers for up to 1 week, or freeze for up to 2 months. Thaw in the fridge or at room temperature until just soft enough to bite.