Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy. Mix in egg, then molasses, spices, baking soda, and salt. Reduce speed to low. Gradually mix in flour. Divide dough into quarters; shape into disks. Wrap in plastic. Refrigerate until firm, about 3 hours (up to 2 days).
Preheat oven to 375. Transfer one disk of dough at a time to a lightly floured work surface. Roll out to 1/4 inch thick. Cut out shapes using a set of 11 fluted round cookie cutters in graduated sizes ( 3/4 inch to 3 1/2 inches). You will use each cutter once for every tree. Using a 1 1/4-inch star cutter, cut out 10 stars. Arrange cookies by size on parchment-lined baking sheets. Refrigerate 20 minutes.
Bake until firm, about 6 minutes for small cookies and 10 minutes for large. Let cool 5 minutes. Transfer cookies to wire racks, and let cool completely.
Assemble trees: Put frosting into a pastry bag fitted with a small plain tip (such as Ateco #12). Pipe a dime-size dot of frosting in center of second-largest cookie; sandwich with largest cookie. Repeat in descending pairs with larger cookies on top to make 5 sandwiches. Pipe a scalloped 1/4-inch-thick border of frosting on top of larger cookie of largest sandwich. Top with the second-largest sandwich, larger cookie up, and repeat to make a tree of 5 sandwiches. Top with frosting, then smallest cookie, then more frosting and a star (standing upright). Cookie trees can be refrigerated up to 6 hours (they will soften). Dust with confectioners' sugar before serving.