Preheat oven to 350°F. Butter a 12-inch cookie pan or cast iron pan, line with a 12-inch round piece of parchment and set aside.
Place flour, baking powder and baking soda in a medium bowl and whisk to combine. Place brown sugar, granulated sugar and salt in the bowl of a stand mixer fitted with a paddle attachment. Mix on low until combined.
Add melted butter and beat until combined. Beat in egg and vanilla, scraping down sides of bowl as needed.
With the mixer on low, slowly add dry ingredients and mix until just combined. Mix in 1 cup chocolate chips. Scoop chunks of dough into the pan and use your fingers to press it around into a smooth, even layer.
Bake for 20-22 minutes, or until golden on the edges but still slightly soft in the center. Run a knife around the edge of the cookie and cool completely. Leave in the pan, if desired, or use a large plate to carefully invert the cookie cake and remove it from the pan.
Decorate a 2-inch edge of the cooled cookie cake with green buttercream frosting to resemble a wreath and top with pearl sprinkles.
Place butter in the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed to soften, about 30 seconds.
Add the powdered sugar, attach splash guard and mix on low speed until it just comes together, about 3 minutes (it will look very powdery at first). Scrape down bowl as needed.
Turn mixer on high, immediately add vanilla extract and hot water, and mix until just combined, about 20 seconds. Beat in green food coloring. Frosting will be very stiff for piping. Thin with heavy cream, a teaspoon at a time, to reach desired consistency.
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