Instructions

• Preheat oven to 350°F (175°C). Line a rimmed baking sheet with parchment paper and lightly butter the parchment and the rim of the 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) softened butter with 1 cup (13g) packed brown sugar and 1 cup (198g) granulated sugar until light and fluffy. Scrape down the bowl, then beat in 2 large (100g) room-temperature eggs and 1 tablespoon (14g) vanilla extract until combined. Add the dry ingredients and mix just until combined. Stir in 2 cups (340g) chocolate chips. Press this cookie dough evenly into the bottom of the prepared baking sheet.
• Make the brownie layer: In a medium saucepan over low heat melt 1 cup (226g) butter. Stir in 1 cup (84g) unsweetened cocoa powder, 1/4 teaspoon (0.6g) espresso powder, and 1/2 teaspoon (3g) kosher salt until smooth, then remove from heat. Stir in 1 cup (213g) packed brown sugar and 1 cup (198g) granulated sugar until combined. Add 4 large (200g) room-temperature eggs one at a time, stirring after each until just blended. Gently fold in 1 cup (120g) all-purpose flour, 2 teaspoons (9g) vanilla extract, and 1 cup (170g) chocolate chunks; fold in 1 cup (109g) chopped pecans if using. Pour and spread the brownie batter evenly over the pressed cookie dough layer.
• Bake: Bake in the preheated 350°F oven until the brownie layer is set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter), about 20–25 minutes. Start checking at 20 minutes. Remove from the oven and place the pan on a wire rack to cool until warm or room temperature.
• Make the warm chocolate sauce while the bars cool: In a small saucepan melt 3 tablespoons (42g) butter over low-medium heat. Add 1/4 cup (50g) sugar, 2 tablespoons (15g) all-purpose flour, and 1/3 cup (28g) unsweetened cocoa powder and stir to combine. Gradually whisk in 2 cups (454g) whole milk, then cook over low-medium heat, stirring constantly, until the sauce thickens and is smooth. Remove from heat and keep warm.
• Assemble the sundaes: Cut the cooled cookie-brownie tray into squares. Place one square in a shallow bowl, sundae glass, or cup. Top with a scoop of the chocolate ice cream (from the 1 pint/264g). Drizzle with the warm chocolate sauce and serve immediately.