1. Preheat oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.

2. For the Brownie Layer: In a large bowl, cream together butter and granulated sugar until light and fluffy. Beat in eggs one at a time, then vanilla extract. In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually add dry ingredients to wet ingredients, mixing until just combined. Stir in 1 cup chocolate chips. Spread evenly into the prepared pan.

3. For the Cookie Layer: In a medium bowl, cream together softened butter, brown sugar, and granulated sugar until smooth. Beat in egg yolk and vanilla extract. In a separate bowl, whisk together flour, baking soda, and salt. Gradually add dry ingredients to wet ingredients, mixing until just combined. Stir in ½ cup chocolate chips.

4. Assemble Brookies: Drop spoonfuls of cookie dough over the brownie layer. Using a knife or spoon, gently swirl the cookie dough into the brownie batter. (Optional: Divide cookie dough and color portions with food coloring before swirling for a Halloween effect).

5. Bake for 30-35 minutes, or until a toothpick inserted into the brownie layer comes out with moist crumbs. Do not overbake.

6. Let cool completely in the pan on a wire rack before cutting into squares. Decorate with Halloween sprinkles if desired.