Preparation Steps

• Preheat the oven to 335°F. Grease and line two 8-inch round cake pans with parchment on the bottoms. Set out sour cream, buttermilk, and eggs to come to room temperature if they aren’t already.
• In a small bowl, rub the lemon zest into the granulated sugar with your fingertips for 30–60 seconds until the sugar is fragrant and slightly damp. In a separate bowl, whisk together the flour, baking powder, and salt.
• Using a stand mixer with the whisk (or a hand mixer), beat the lemon sugar and eggs on medium-high until pale and thick, about 2–3 minutes. With the mixer on low, stream in the oil, then mix in the vanilla bean paste and sour cream until just combined. Stir in 1/4 cup fresh lemon juice.
• Add the dry ingredients in three additions, alternating with the buttermilk in two additions, beginning and ending with dry. Mix on low only until the flour disappears. Let the batter rest 5 minutes to relax the air bubbles.
• Divide batter evenly between the pans and smooth the tops. Bake 26–30 minutes, or until the centers spring back and a toothpick tests clean. Cool in pans 10 minutes, then turn out onto racks and cool completely. For cleaner layers, chill the cakes 20–30 minutes before slicing.
• Make the frosting: Beat the softened butter on medium-high for 2 minutes until light. Add the cream cheese and a pinch of salt; beat until completely smooth, 1–2 minutes, scraping the bowl. Mix in half the powdered sugar on low. Add lemon curd, limoncello (or milk), and lemon zest; mix until combined, then add remaining powdered sugar and beat 2 minutes until fluffy. If soft, chill 10 minutes before using.
• Torte and fill: Using a long serrated knife, split each cake horizontally to make four thin layers total. Place one layer on a board. Spread a thin veil of frosting across the surface, then create a shallow frosting ditch about 1/2 inch from the edge. Spoon 1/4–1/3 cup lemon curd into the center and nudge it to the ditch. Repeat with the next two layers. Top with the final layer.
• Set the stack in the refrigerator for 10–15 minutes to help the curd settle. Apply a thin crumb coat of frosting and chill another 10 minutes. Finish with a smooth or swoopy final coat. Garnish with thin lemon slices if desired.
• Tip: If your kitchen is warm, keep the jar of lemon curd chilled and the cake layers briefly frozen (10 minutes) before assembly to prevent the filling from squeezing out.