Step 1: Preheat your oven to 350°F (175°C) for even baking.
Step 2: Grease your baking pan with butter or line with parchment paper.
Step 3: In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Beat until smooth and creamy.
Step 4: Add the egg, egg yolk, and vanilla extract to the bowl. Mix until well combined.
Step 5: In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add to the wet mixture, mixing until just combined.
Step 6: Gently fold in the chocolate chips.
Step 7: Spread the batter evenly into the prepared pan and bake for 25-30 minutes or until edges are golden.
Step 8: Prepare the frosting by beating together butter, powdered sugar, cocoa powder, milk, and vanilla until smooth.
Step 9: Once cooled, frost the cookie cake and add sprinkles for decoration.
