Preheat oven to 350 degrees F. Line three 6-inch cake round pans with parchment paper. Lightly grease and set aside. Note: I used 8-inch round pans from Michael's and just doubled the recipe.
In a large bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter. Add sugar and whip until smooth. Add eggs and continue to mix on medium speed until incorporated. Incorporate sour cream and mix. Scrape down bowl as needed to ensure thorough mixing.
Add flour mixture in three additions, alternating with milk. Begin and finish with flour mixture. Add lemon juice and lemon zest and mix until well incorporated.
Divide batter evenly among the three prepared cake pans. Level batter using a mini offset spatula, or the back of spoon. Bake for 35-40 minutes until a toothpick inserted in center of cake comes out clean. Let cake cool in pan for 5 minutes, then run a mini offset spatula around the cake and unmold. Let cake cool to room temperature on wire rack.
In a heavy bottomed sauce pot, add butter and rosemary. Melt over medium high heat and bring to a boil. Allow to boil for 1 minutes. Remove from heat and allow to cool. Allow rosemary to steep for 60-90 minutes until butter is cool to the touch. Strain out rosemary and keep butter chilled in the fridge until ready to use.
Let chilled rosemary infused butter soften at room temperature before use. Using a hand held mixer or a stand mixer fitted with a paddle attachment, cream butter. Add confectioners' sugar, salt, and cream. Mix until buttercream is smooth. If buttercream is too soft, add additional confectioners' sugar 1 Tbsp at a time until desired consistency is achieved.
Fit a decorating bag (piping bag) with a small round tip. Fill the bag with about 1/2 cup of buttercream. Set aside.
If necessary, level cake layers. Place one cake round on a cake turntable. Spread a thin layer of buttercream on cake using a small offset spatula. Using the filled piping bag, pipe a small border of rosemary buttercream along the perimeter of the cake round. This will serve as a barrier when you filled the cake with lemon curd.
Add enough lemon curd to fill the buttercream boundaries. Place second cake layer on top. Repeat filling cake layer with buttercream and lemon curd. Place the final third layer on top. This layer will not have lemon curd
Spread buttercream around the cake to create a "crumb coat" layer to seal in any crumbs. Chill cake in the fridge for 15-20 minutes to set frosting. If desired, generously coat the chilled cake with additional buttercream. Chill cake if not serving immediately. Decorate cake with fresh rosemary sprigs and crushed candied pecans. Dust top of the cake with sifted powdered sugar. Let cake sit at room temperature for at least 30 minutes before serving to allow cake to soften.