Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper. If you have a Christmas tree-shaped cake pan, you can grease it well, but a flat sheet works perfectly for hand-shaping.
In a large mixing bowl, beat the softened butter with the brown sugar and granulated sugar until the mixture is light and fluffy. This usually takes about 2 to 3 minutes with an electric mixer at medium speed.
Add the egg and vanilla extract to the butter mixture. Beat well until everything is fully incorporated and the texture is smooth.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt. Slowly add the dry ingredients to the wet mixture, stirring until a thick dough forms. Fold in the chocolate chips by hand using a spatula.
Press the cookie dough onto your prepared baking sheet. Shape it into a large triangle to resemble a tree, making sure it is about 1/2 inch thick. Bake for 20 to 25 minutes, or until the edges are golden brown. Let the cookie cake cool completely before decorating.
Once cooled, pipe the green frosting onto the cookie in a zig-zag pattern or small dollops to look like branches. Add sprinkles as “ornaments” and place a star candy or a large yellow sprinkle at the very top.