To Make the Dough:
In bowl of electric mixer fitted with paddle attachment, mix butter and sugar together until just incorporated. Do not over mix. Add egg and vanilla extract. Mix on low speed, scraping sides of bowl as needed.
In medium bowl, whisk together flour and salt. Add flour to butter mixture. Mix on low speed until dough forms and clumps around paddle. If dough does not come together and is crumbly, add ice cold water 1 tablespoon at a time until dough clumps.
To Marble the Dough:
Remove half of dough from bowl and set aside. Add green gel food coloring a little at a time to dough in mixing bowl until a consistent green color is achieved. Add additional flour 1 tablespoon at a time if dough becomes sticky.
Roll each piece of dough flat between two sheets of parchment paper. Remove top piece of parchment from each and press two pieces of dough together.
Cut stacked dough into strips and knead each strip together so that the pieces are slightly marbled. Place pieces on sheet of parchment paper and cover with another sheet.
To Bake the Cookies:
Roll flat to about ¼-inch thick with rolling pin. Chill dough at least 30 minutes before cutting into shapes.
Preheat oven to 350°F. Line two baking sheets with parchment paper. Use large (about 8-inch) tree-shaped cookie cutter to cut 3 shapes. Use knife to cut bottoms of trees flat (remove ‘tree trunk’ if applicable). Remaining dough can be used to cut smaller shapes.
Bake 15-20 minutes for 8-inch shapes and 10-12 minutes for 5-inch shapes. Cookies should be lightly golden around edges. Let cookies cool on sheets 5 minutes. While still hot, cut two trees in half down center. Transfer to wire cooling rack to cool completely.
To Decorate & Assemble 3D Sugar Cookie Trees:
Use fine-tipped artists’ brush to place dots of corn syrup on trees. Place dragees on top of dots and let stand until firmly set, about 1 hour.
Transfer melted almond bark to piping bag and pipe line down center of whole tree. Press two cut tree pieces, angled into candy. Let stand until dry, about 10 minutes. Repeat process with opposite side of tree.
Place cookie tree on cake board or serving platter. Dust lightly with powdered sugar.