Preheat oven to 350º F. Line a baking sheet with parchment paper and lightly coat the paper and the rim of the baking sheet with butter.
For the Chocolate Chip Cookie Layer
Whisk together the dry ingredients (flour, cornstarch, salt, baking soda) for the chocolate chip cookie dough. Cream together the butter and sugars until light and fluffy. Scrape down the sides of the bowl and incorporate the eggs. Mix until well combined. Add the dry ingredients to the butter and sugar mixture and mix until well combined. Press the dough into the bottom of a baking sheet.
For the Brownie Layer
Melt butter in a large saucepan over low heat. Stir in cocoa powder, espresso powder, and salt until well combined. Remove from heat. Stir in both sugars and then the eggs, one at a time, mixing until just blended. Gently stir in the flour and vanilla, along with any add-ins you are using. Pour over the chocolate chip cookie dough layer. Top with the chocolate chunks.
Bake chocolate chip brownies until a toothpick inserted in the center comes out clean, about 20 minutes. Remove from the oven and set aside.
For the Ice Cream
Remove from the freezer and allow to thaw slightly as you prepare your chocolate sauce.
For the Chocolate Sauce
Melt butter in a small saucepan or cast iron skillet. Add sugar, flour, and cocoa. Add milk. Cook over low-medium heat, stirring constantly until the chocolate sauce has thickened.
To Assemble
Cut a square of the cookie brownie and place in a shallow bowl, sundae glass, or cup. Scoop one scoop of ice cream and place on top of cookie brownie bottom. Drizzle with warm chocolate sauce and serve immediately.
Notes
Recipe Substitutions
Chocolate Chip Cookie Dough- use your favorite store-bought dough.
Brownie - use your favorite store-bought brownie mix.
Chocolate Ice Cream - use your favorite store-bought ice cream.
Chocolate Syrup - use your favorite store-bought chocolate syrup.