Butter the sides and bottoms of two 9-inch round cake pans and line the bottoms with parchment paper disks cut to size. Butter the parchment paper and dust with flour.
Combine the butter and the milk in a medium saucepan and cook over low heat until the butter is completely melted. Set aside.
Sift the flour and the baking powder over a medium mixing bowl.
In the bowl of an electric mixer, or a stand up mixer, whisk the eggs with the salt. Slowly add in the sugar, then the lemon juice and lemon zest, while continue whisking at medium-high speed until well mixed, about 3 minutes.
Slowly add the warm milk and butter mixture, while whisking gently.
Remove the bowl from the stand up mixer and add the flour mixture in a few additions, whisking until completely incorporated. The batter will be slightly liquid. Divide the batter evenly into the two cake pans, tap the pans gently on the counter to level their content, and place in the oven. Bake until the cakes have risen and their surface is golden, about 25-30 minutes (if you’re not sure of their doneness, insert a toothpick in the center and make sure it comes out clean).
Use a small knife to detach the cakes from the sides of the pan, then invert the cakes onto cooling racks. Immediately re-invert the cakes and let cool completely.
Make the buttercream:
Sift the icing sugar over a bowl large enough to hold it.
In the bowl of a stand up mixer, beat the butter for about 2 minutes. Stop the mixer and add 1/2 cup of icing sugar, then beat on low speed first until incorporated. Continue this way until all the sugar is incorporated. Add the vanilla extract and the lemon curd and beat until all ingredients are fully incorporated and the buttermilk is smooth and light.
Decorate the gingerbread cookies:
Mix the icing sugar with the water to make the royal icing (adjust with more sugar if necessary so the icing is not too runny). Fit a plastic icing bag with a number 1 or number 2 icing tip, fill with the royal icing, and with a gentle but steady hand, decorate the cookies to your liking.
Assemble the cake:
Remove the paper disks from the cake bottoms. With a serrated knife, cut off the tops of the cakes until leveled. Then slice each cake horizontally into two rounds. You will end up with 4 equal cake rounds. Place one cake on a plate, cut-side up. Spread about 3/4 cup of frosting with an offset spatula. Top with a second cake, cut-side down. Repeat the process with the frosting then top with a third cake, cut-side up. Spread another 3/4 cup frosting and top with the last cake layer, cut-side down. Spread the remaining frosting evenly over the top and sides of the cake, using a frosting blade to evenly spread the frosting over the sides of the cake. Remove excess frosting from the sides of the cake to give it the naked effect.
Decorate with the cookies and with the rosemary sprigs.
Notes
The cakes can be wrapped in plastic and assembled the next day
For the trees, place the rosemary sprigs upside down
Spread a bit of buttercream on the back of each cookie so they stick to the cake more securely