Slice each cooled pumpkin cake into two layers. Place one cake round on cake board (or serving plate). Spread a layer of frosting over cake layer. Sprinkle a handful of chopped dark chocolate over frosting.
Place second cake layer on top. Gently press cake layer down to secure. Spread a layer of frosting and add another layer of chopped chocolate. Continue to repeat until fourth cake layer of added on top.
Spread frosting all over cake to seal in the crumbs (crumb coat). Let chill in the refrigerator for at least 15 minutes. If desired, add another layer of buttercream to coat cake. If desired, pipe border along the bottom of the cake. Otherwise, continue to chill cake until ready to serve.
Decorate cake before serving. Use fresh rosemary sprigs, chopped pistachios, and chopped chocolate to garnish the top of cake. Add additional chopped pistachios and chopped chocolate along the bottom of the cake. Place animal toy figurines, if desired. As a final touch, dust cake with powdered sugar to resemble snow.