• Preheat oven to 350℉.
• In a large microwave-safe bowl, melt the butter (½ cup) for 20-30 seconds, or until fully melted.
• Using a spatula, stir in the light brown sugar (½ cup) and granulated sugar (¼ cup) until combined. Add the egg (1 large) and vanilla extract (2 teaspoons), then stir.
• Stir in the baking soda (½ teaspoon), baking powder (¼ teaspoon), and salt (¼ teaspoon). Add the all-purpose flour (1 ½ cups) and stir until combined. Fold in the chocolate chips (1 cup).
• Drop chunks of dough into a greased and parchment-lined 9x13" pan, then use a spatula or your hands (sprayed with nonstick cooking spray) to smooth it out.
• Bake at 350°F for 12-14 minutes, or until it looks slightly brown on top and slightly underbaked in the center. Remove from the oven and place the pan on a cooling rack. Allow the cookie cake to cool completely before removing from the pan.
• To cut into trees, use the diagram below. This is also on page 16 of my cookbook.
  

To Make Frosting:

• With an electric mixer, beat unsalted butter (½ cup) on medium-high speed for 1 minute until smooth and creamy (if using a stand mixer, use the paddle attachment). Gradually add powdered sugar (2 ½ cups), vanilla extract (1 teaspoon), and pinch of salt, mixing on low speed. Add milk one tablespoon at a time (2-3 tablespoons total), then beat on high for 2-3 minutes until creamy. Adjust the consistency with more powdered sugar or milk if needed.
• For decorating, reserve ¼ cup of the frosting and color it yellow. Fit a piping bag with a star tip to pipe the stars, then color the remaining frosting green. Add sprinkles right away, as the frosting sets quickly.