1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper and lightly grease.

2. Make brownie layer: in a medium bowl whisk melted butter, granulated sugar, eggs, and vanilla until smooth.

3. Stir in cocoa powder, flour, salt, and baking powder until just combined. Pour batter into pan and spread evenly.

4. Make cookie dough: in a separate bowl cream softened butter, brown sugar, and granulated sugar until fluffy.

5. Mix in egg yolk and vanilla. Stir in flour, baking soda, and salt until just combined, then fold in chocolate chips.

6. Drop spoonfuls of cookie dough evenly over the brownie layer. Do not smooth completely; leave some peaks.

7. Bake 30–35 minutes until cookie top is golden and a toothpick in the brownie layer comes out with moist crumbs.

8. Cool completely in pan, about 30 minutes. Remove using parchment and transfer to a cutting board.

9. Decorate: sprinkle orange and black sprinkles and candy eyeballs over the top.

10. Cut into 12 bars and serve.