Step 1: Preheat oven to 350 degrees F (175 degrees C). Grease a baking dish with butter.
Step 2 (Prepare the Cookie Layer):
Beat softened butter, light brown sugar, 1/4 cup white sugar, and 1/2 teaspoon vanilla extract together in a large bowl until creamy. Add 1 egg, beat until light and creamy, about 2 minutes. Whisk 1 1/4 cups flour, 1/2 teaspoon salt, and baking soda together in a bowl. Gradually stir flour mixture into butter mixture until dough is combined. Stir chocolate chips into dough. Spread dough into the bottom of the prepared baking dish to cover completely.
Step 3 (Prepare the Brownie Layer):
Stir 1 box Betty Crocker fudge brownies mix, melted butter, and milk together in a bowl, add 1 egg and beat well. Pour brownie batter over cookie dough and spread to cover completely.
Step 4: Bake in the preheated oven until a toothpick inserted into the center of the brookies come out clean, 20 to 25 minutes. Cool completely before cutting into bars.