• Preheat oven to 350℉.
• In a large microwave-safe bowl, melt the ½ cup unsalted butter (1 stick) for 20-30 seconds, or until fully melted.
• Using a spatula, stir in the ½ cup light brown sugar and ¼ cup granulated sugar until combined. Add the 1 large egg and 1 teaspoon vanilla extract, then stir.
• Stir in the ½ teaspoon baking soda, ¼ teaspoon baking powder, and ¼ teaspoon salt. Add the 1 ½ cups all-purpose flour and stir until combined. Fold in the 1 cup semi-sweet chocolate chips.
• 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 ½ cup unsalted butter on medium-high speed for 1 minute until smooth and creamy (if using a stand mixer, use the paddle attachment). Gradually add 2 ½ cups powdered sugar, 1 teaspoon vanilla extract, and pinch of salt, mixing on low speed. Add 2-3 tablespoons milk, one tablespoon at a time, 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.