Start by gathering all your ingredients. I find it helpful to place them in separate bowls for each layer; it keeps things organized and ensures I don’t forget anything essential.

Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). While your oven heats up, grease a 9x13-inch baking dish to prevent sticking.

In a large mixing bowl, beat together the softened butter, light brown sugar, granulated sugar, and vanilla extract until creamy. This usually takes about two minutes with an electric mixer. Add in the egg and beat until the mixture is light and fluffy.

In another bowl, whisk together the flour, baking soda, and salt. Gradually stir this dry mixture into your butter mixture until combined. Now, fold in those lovely chocolate chips.

Spread this cookie dough evenly in your prepared baking dish, making sure to cover the bottom.

Now, let’s move on to the brownie layer. In a separate bowl, stir together the granulated sugar, melted butter, and vanilla extract. Next, beat in the eggs one at a time, making sure to mix well after each addition.

Sprinkle in the cocoa powder, flour, baking powder, and salt. Stir until just combined; the batter will be thick and luscious. If you’re adding nuts for crunch, now’s the time to fold them in.

Pour this brownie batter over the cookie layer, spreading it out to cover completely.

Pop your baking dish in the preheated oven. Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. It’s so tempting to peek!

Once done, remove your dish from the oven and allow it to cool completely in the pan. This step is crucial; cooling helps the layers to set properly, making it easier to cut into bars. Once they're cool, slice into 20 beautiful squares, and you are ready to enjoy!