Preheat your oven to 350°F (175°C). Line a 9-inch cake pan or springform pan with parchment paper, butter the parchment, and dust the pan with flour. Shake out any excess flour.
Butter
In a medium bowl, whisk the flour, salt, and baking soda together. Set aside.
1 ⅓ cups all-purpose flour, ¼ teaspoon salt, ¼ teaspoon baking soda
Using a stand mixer or handheld mixer, cream the butter and sugar together until fluffy. Add the egg, then the vanilla extract, and mix until smooth. Gradually add the flour mixture and mix until the dough forms. Add green food gel until the desired shade is reached.
10 tablespoons unsalted butter, ⅔ cups granulated sugar, 1 large egg, 1 ½ teaspoon vanilla extract, Green food gel
Transfer the dough to the prepared pan and spread it evenly with a palette knife. Bake for 14-15 minutes until the edges are golden, but the center remains soft. Once out of the oven, gently press down any puffed edges with the bottom of a glass. Let the cookie cool completely in the pan.
Run a knife around the edge to loosen the cookie. Invert onto a cutting board if using a cake pan or remove the ring if using a springform pan. Cut into 8 triangles and trim the rounded ends to make tree shapes.