Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides so you can lift the bars out later for easy slicing.
Spread the all-purpose flour on a baking sheet and bake at 350°F (175°C) for 5–7 minutes, stirring once, until it reaches 165°F. Let it cool completely before using.
1 1/2 cups all-purpose flour
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together with a hand mixer or stand mixer on medium speed for 2–3 minutes, until light and fluffy.
1/2 cup unsalted butter, 3/4 cup light brown sugar, 1/4 cup granulated sugar
Mix in the fine sea salt and vanilla extract. Add the milk and mix on low speed until smooth; the mixture will look slightly loose.
1/4 teaspoon fine sea salt, 2 teaspoons pure vanilla extract, 1/4 cup whole milk
Sift or sprinkle the cooled, heat-treated flour over the butter mixture. Mix on low speed just until incorporated. The dough should be thick but soft. If it seems too dry, mix in 1–2 teaspoons more milk.
1 1/2 cups all-purpose flour, 1/4 cup whole milk
Fold the mini chocolate chips into the dough with a spatula until evenly distributed, being careful not to over-mix or melt the chips.
1 cup mini chocolate chips
Transfer the cookie dough to the prepared pan. Press it firmly into an even layer using clean hands, a spatula, or the bottom of a measuring cup with a piece of parchment on top.
Refrigerate the dough layer for 20–30 minutes so it firms up slightly before adding the chocolate topping.
In a microwave-safe bowl, combine the semi-sweet chocolate chips and butter or coconut oil. Microwave in 20–30 second bursts, stirring between each, until melted and smooth. Alternatively, melt gently over a double boiler.
1 1/2 cups semi-sweet chocolate chips, 2 tablespoons butter or coconut oil
Pour the melted chocolate mixture over the chilled cookie dough layer. Spread evenly with an offset spatula or the back of a spoon. If desired, sprinkle flaky sea salt over the top.
flaky sea salt
Refrigerate the pan for at least 1 hour, or up to 2 hours, until the chocolate is set and the bars feel firm.
Use the parchment overhang to lift the slab out of the pan onto a cutting board. Slice into 16–20 bars with a sharp knife, wiping the blade between cuts for clean edges. Serve chilled or let sit at room temperature for 5–10 minutes for a softer texture.