In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. This should take about 3-5 minutes. Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the flour, cornstarch, kosher salt, and baking soda. Gradually add the dry ingredients to the butter mixture, stirring until just combined. Do not overmix. Fold in the chocolate chips and chunks, ensuring they are evenly distributed.
Pour the brownie batter into a greased 9x13-inch baking pan. Smooth the top with a spatula and bake in a preheated oven at 350°F (175°C) for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan.
In a saucepan over medium heat, melt the butter for the sauce. Once melted, stir in the brown sugar and granulated sugar until combined. Add the cocoa powder, flour, and espresso powder, mixing well. Gradually whisk in the milk, stirring constantly until the mixture thickens and bubbles. Remove from heat and stir in the vanilla extract.
Once the brownies are fully cooled, cut them into squares. Place a brownie square in a bowl, top it with a scoop of chocolate ice cream, and drizzle generously with the warm chocolate sauce. If desired, sprinkle with chopped pecans for added texture.
Serve the Chocolate Chip Brownie Sundae immediately, allowing everyone to dig into this decadent treat. Enjoy the combination of warm brownies, cold ice cream, and rich chocolate sauce!
