Preheat your oven to 350 F (175 C). Grease two 8-inch cake pans and line the bottoms with parchment rounds. Lightly grease the parchment and dust with flour, tapping out excess.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until uniform.

In the bowl of a stand mixer fitted with the paddle attachment, cream the room temperature butter and sugar on medium speed for 3 to 4 minutes, until pale and fluffy.

Add eggs one at a time, mixing for 20 to 30 seconds after each addition. Scrape the bowl down as needed. Mix in the vanilla extract.

Reduce mixer speed to low. Add one third of the dry ingredients, then half of the buttermilk, mixing just until incorporated. Repeat, ending with the remaining dry ingredients. Fold in the sour cream with a spatula if using.

Divide the batter evenly between your prepared pans. Smooth the tops with an offset spatula. Bake 25 to 30 minutes for 8-inch pans, or 18 to 22 minutes for 6-inch pans.

Let the cakes cool in the pans for 10 minutes, then invert onto a cooling rack and remove parchment. Cool completely before frosting.

In a clean bowl, beat softened butter on medium speed until smooth and slightly fluffy. Gradually add the powdered sugar, 1 cup at a time, mixing on low until combined. Add lemon juice, lemon zest, and a pinch of salt.

If layers are domed, level them with a serrated knife or cake leveler. Place the first layer on your cake board or plate, spread a thin layer of buttercream, and add a light smear of jam or curd if using.

Apply a thin crumb coat of buttercream around the cake and refrigerate 15 to 20 minutes to set.

Apply the final generous layer of buttercream, smoothing with an offset spatula and bench scraper until the sides are neat.

Refrigerate the cake for 20 to 30 minutes to set the frosting before slicing. Bring to room temperature 30 minutes before serving.