• Preheat the oven to 190° C/375° F in fan mode.

For the cookies:

• Place the flour, salt, and baking soda in a bowl and mix with a spoon. Set aside.
• In a mixer bowl, place the sugar and butter and blend with the paddle attachment on medium speed until the butter is fluffy.
• Add the egg and vanilla extract and blend until absorbed.
• Add the flour mixture to the mixer bowl and blend until homogenized.
• Divide the cookie mixture into 3 pieces and roll each piece into a log. The first log should be 2 cm/about 1-inch thick, the second 3 cm/1 ¼-inches thick, and the third 4 cm/1 ½ inches thick.
• Cut each log into 1 cm (about 1/3 of an inch) thick pieces and place them on a baking tray lined with parchment paper, leaving a gap between them.
• Bake for 12 minutes until golden. Remove from the oven and set aside to cool.

For the frosting:

• Place the butter, powdered sugar, and milk in the mixer bowl. Beat with the whisk attachment on medium speed until the mixture is fluffy and homogenous.
• Add as much green food coloring as desired, whisk again, and transfer to a pastry bag with a curly nozzle.

Assembly:

• Take a large cookie and cover it with green frosting. Place a smaller cookie on top and cover it with green frosting. Place one of the smallest cookies on top and cover it with green frosting, creating a beautiful tree.
• Repeat the same process for the rest of the cookies, then top each one with a yellow chocolate, and serve.