Instructions

• Preheat the oven to 350°F (177°C). Line a rimmed baking sheet with parchment paper and lightly butter the parchment and the rim of the baking sheet.
• Make the chocolate chip cookie base: In a medium bowl, whisk together 3 cups (360g) all-purpose flour, 2 teaspoons (7g) cornstarch, 3/4 teaspoon (2g) kosher salt, and 1 teaspoon (6g) baking soda. In a large bowl, cream 1 cup (226g) butter (softened) with 1 cup (13g) packed brown sugar and 1 cup (198g) granulated sugar until light and fluffy. Scrape down the bowl, add 2 large eggs (room temperature) and mix until incorporated, then stir in 1 tablespoon (14g) vanilla extract. Add the dry ingredients and mix until combined. Fold in 2 cups (340g) chocolate chips. Press the dough into an even layer in the bottom of the prepared baking sheet.
• Make the brownie layer: In a large saucepan, melt 1 cup (226g) butter over low heat. Stir in 1 cup (84g) unsweetened cocoa powder, 1/4 teaspoon (0.6g) espresso powder, and 1/2 teaspoon (3g) kosher salt until combined. Remove the pan from the heat and let the mixture cool for 1–2 minutes. Stir in 1 cup (213g) brown sugar and 1 cup (198g) granulated sugar until dissolved. Add 4 large eggs (room temperature), one at a time, mixing until just blended after each addition. Gently stir in 1 cup (120g) all-purpose flour and 2 teaspoons (9g) vanilla extract. If using, stir in 1 cup (109g) chopped pecans.
• Assemble before baking: Pour the brownie batter evenly over the pressed cookie dough in the baking sheet. Evenly sprinkle 1 cup (170g) chocolate chunks over the top of the brownie batter.
• Bake: Bake in the preheated oven until a toothpick inserted into the center comes out with a few moist crumbs (or mostly clean), about 20 minutes. Remove from the oven and let the pan cool on a wire rack until warm or room temperature (cooling time will vary).
• While the pan cools, make the chocolate sauce: In a small saucepan, melt 3 tablespoons (42g) butter over medium-low heat. Whisk in 1/4 cup (50g) sugar, 2 tablespoons (15g) all-purpose flour, and 1/3 cup (28g) unsweetened cocoa powder until smooth. Gradually whisk in 2 cups (454g) whole milk. Cook over medium-low heat, stirring constantly, until the sauce comes to a simmer and thickens to a pourable consistency, about 3–5 minutes. Remove from heat and keep warm.
• Serve: Cut the cookie-brownie slab into squares. Place one square in a shallow bowl, sundae glass, or cup. Top each square with a scoop of chocolate ice cream (from the 1 pint/264g). Drizzle with warm chocolate sauce and serve immediately.