Preheat oven to 375F. Line the baking sheet with parchment.

Mix butter, brown sugar, and granulated sugar with mixer until blended.

Add molasses, egg, and vanilla; mix until incorporated.

In another bowl, whisk flour, cinnamon, ginger, all spice, and salt. Add to wet and mix until crumbly dough forms.

Roll dough between parchment sheets to ¼-inch thickness.

Cut out 3D tree cookies out of the dough, slightly thinner than the interlock slot. Transfer to a baking sheet. Knead leftover dough back into the ball and roll out between parchment until it’s all used.

Bake for 9–10 min, until set. Cool 2–3 min, then transfer to a wire rack. You’ll want to test the cookies to see if they lock together but wait until they are FULLY COOLED, or they will likely crumble.

For frosting: Beat water, meringue powder, and vanilla until soft peaks form. Add powdered sugar; mix until thick. Tint if desired.

Frost cookies, allow to dry fully. This will take at least 6 hours, but overnight is safest. Then assemble into trees by gently pushing the bottom-cut tree onto a top-cut tree.

If the trees are having trouble standing after assembly, add a bit of royal frosting to the gaps in the locking areas as you would with a craft project. Lean on a clean glass or jar until the icing dries.