Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to act as handles. Lightly mist the parchment with nonstick spray if desired to prevent sticking.

Preheat the oven to 300°F (150°C). Spread the all-purpose flour in an even layer on a baking sheet. Bake for 5–7 minutes, stirring once halfway through, until it reaches at least 165°F (74°C) on an instant-read thermometer. Let the flour cool completely before using so it doesn’t melt the butter. Alternatively, microwave the flour in 30-second bursts, stirring between each, until it reaches temperature, then spread out to cool.

In a large mixing bowl, beat the softened butter, packed light brown sugar, and granulated sugar together for 2–3 minutes, until light and fluffy. Use a stand mixer with the paddle attachment or a hand mixer, scraping down the sides of the bowl as needed.

Beat in the vanilla extract and fine sea salt until fully combined. Taste a tiny bit and add a small extra pinch of salt if you prefer a slightly saltier, bakery-style cookie dough flavor.

Once the heat-treated flour is completely cool, gradually add it to the butter mixture on low speed. Mix just until no streaks of dry flour remain. The dough will look thick and a bit crumbly at this stage.

Add 2 tablespoons of milk or heavy cream and beat until the dough comes together and looks like soft cookie dough. If it still seems dry or sandy, add up to 1 more tablespoon, a small splash at a time, beating between additions. The dough should hold together easily and feel smooth, not dry.

Using a spatula, fold the mini chocolate chips into the dough until evenly distributed. If your kitchen is warm and the chips begin to melt, chill the bowl in the refrigerator for 5–10 minutes before continuing.

Transfer the cookie dough to the prepared 8x8-inch pan. Use clean hands, a spatula, or the back of a measuring cup to press it into an even, smooth layer. If the dough sticks, lightly dampen your fingertips or place a piece of parchment over the dough and press through the parchment.

For extra-neat layers, chill the pressed cookie dough layer in the refrigerator for 20–30 minutes before adding the chocolate topping, especially if the dough feels warm or very soft.

In a microwave-safe bowl, combine the semisweet chocolate chips and butter or coconut oil. Microwave in 20–30 second bursts, stirring well after each, until completely melted and smooth. Alternatively, melt together in a heatproof bowl set over a small pot of simmering water (double boiler method).

Pour the melted chocolate mixture over the cookie dough layer. Tilt the pan or use an offset spatula to spread the chocolate into a smooth, even layer that fully covers the surface. While the chocolate is still warm, optionally sprinkle a few extra mini chocolate chips, crushed pretzels, sprinkles, or a pinch of flaky sea salt on top.

Refrigerate the pan for at least 2 hours, or until the chocolate is firm and the cookie dough has set. For a faster option, chill in the freezer for about 1 hour, then let sit at room temperature for a few minutes before slicing.

Use the parchment overhang to lift the chilled slab from the pan onto a cutting board. Run a sharp knife under hot water, dry it, then slice into 16–20 squares, wiping the blade clean between cuts for neat edges. Serve chilled or at cool room temperature.