Preheat oven to 350 degrees. Coat three 6- inch round cake pans with cooking spray. Line bottoms with parchment; coat parchment with cooking spray. Dust with flour; tap out excess. Set aside.
Stir boiling water and baking soda in a small bowl; set aside. Sift together flour, spices, salt, and baking powder into a large bowl; set aside.
Put butter, sugar, and zest into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until pale and fluffy, about 3 minutes. Reduce speed; mix in molasses, liqueur if desired, orange juice, corn syrup, and baking soda mixture. Mix in flour mixture, then eggs and vanilla. Stir in nuts.
Divide batter among prepared pans. Bake until a cake tester comes out clean, 30 to 35 minutes. Let cool on racks 15 minutes. Invert to unmold onto racks; remove parchment. Reinvert; let cool completely. Store cakes at room temperature, wrapped well in plastic, up to 1 day.
Trim tops of cakes flat. Spread 1/2 cup frosting on cut side of 1 layer. Top with a second layer; spread with 1/2 cup frosting. Top with final layer, cut side down. Spread remaining frosting over top and sides, smoothing with an offset spatula.
Place 6 or 7 green chocolate leaves on top of cake in a circle with tips facing out, overhanging sides slightly. Place 12 to 15 pink leaves on top of green leaves, leaving a hole in center. Roll marzipan into six to eight 1/8-inch balls; taper one end of each into a point. Place marzipan buds in center, point side down. Serve immediately or refrigerate up to 8 hours. Bring to room temperature before serving.