1. Preheat the oven to 350F. Butter two 9x2" round cake pans; line the bottoms with parchment paper. Butter parchment, and dust with flour, tapping out excess; set aside. Into a medium bowl, sift together the flours, baking powder, and salt; set aside. In a small bowl, combine sour cream with lemon zest and juice; set aside.
2. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium-high speed until fluffy, about 1 minute. Add sugar and continue beating until light and fluffy, 4 to 5 minutes, scraping down the sides of the bowl as needed. Add eggs, one at a time, beating for 1 minute after each addition.
3. With the mixer on lowest speed, add the flour mixture in four parts, alternating with the sour cream mixture and beginning and ending with the flour mixture; beat until just combined, being careful not to overmix.
4. Divide batter between prepared pans, and smooth with an offset spatula. Bake, rotating pans halfway through, until cakes are golden and pull away from sides of pan, and a cake tester inserted into the centers comes out clean, 30 to 35 minutes. Transfer pans to a wire rack to cool 20 minutes. Run a knife or thin metal spatula around the edges of the pans. Invert cakes onto the rack; peel off the parchment. Reinvert cakes and let them cool completely, top sides up.
5. Using a serrated knife, trim tops of the cakes to make level. Slice each layer in half horizontally into two layers. Place one of the bottom layers on a cake plate, and spread 1/2 cup lemon curd over the top. Repeat with remaining cake layers and curd, leaving final cake layer uncoated. Spread a thin layer of lemon Swiss meringue buttercream over the entire cake to seal in crumbs; refrigerate until well chilled, about 30 minutes.
6. Using an offset spatula, spread entire cake with remaining buttercream. Refrigerate 30 minutes before decorating with lemon curd: Using a paper cornet filled with curd, pipe dots of varying sizes on top of the cake. Starting at the bottom, pipe vertical lines of varying lengths up the side of the cake. Cake can be kept in the refrigerator, covered with a cake dome, for up to 3 days.

Lemon Curd Directions:

1. Combine yolks, lemon zest, and sugar in a heavy-bottom saucepan; whisk to combine. Cook over medium-high heat, stirring constantly with a wooden spoon (be sure to scrape the sides of the pan), until the mixture is thick enough to coat the back of the spoon, 8 to 10 minutes, and registers 160F on an instant-read thermometer.
2. Remove saucepan from heat. Add salt and butter, one piece at a time, stirring until smooth. Strain through a fine sieve into a medium bowl. Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin from forming. Refrigerate until chilled and set, at least 1 hour or up to 1 day.

Swiss Meringue Buttercream Directions:

1. In the heatproof bowl of an electric mixer set over a saucepan of simmering water, combine the egg whites and sugar. Cook, whisking constantly, until the sugar has dissolved and the mixture is warm to the touch.
2. Attach the bowl to the mixer fitted with the whisk attachment. Beat the egg-white mixture on high speed until it holds stiff (but not dry) peaks. Continue beating until the mixture is fluffy and cooled, about 6 minutes.
3. Switch to the paddle attachment. With the mixer on medium-low speed, add the butter several tablespoons at a time, beating well after each addition. (If the frosting appears to separate after all the butter has been added, beat on medium-high speed until smooth again, 3 to 5 minutes more.) Stir in 3/4 cup lemon curd with a rubber spatula.

Source: Martha Stewart's Baking Handbook