This all-lemon cake has a tart curd filling, buttercream icing, and candied fruit on the side. Poppy seeds give it subtle crunch.
Preheat oven to 350 degrees. Coat cake pans with cooking spray; cover bottoms with parchment, and coat again. Set aside.
Grate zest of lemons (you should have about 4 tablespoons). Remove pith from each lemon with a sharp knife. Working with 1 at a time, hold lemon over a bowl, and carve out flesh between the membranes, allowing segments to drop into bowl (you should have about 1 cup segments). Squeeze juice into bowl. Remove seeds. Break segments into 1-inch pieces.
Sift flour, sugar, and baking powder into the bowl of an electric mixer. Attach bowl to mixer fitted with the paddle attachment. With mixer on low speed, add creme fraiche. Raise speed to medium. Add eggs, 1 at a time, mixing well after each addition. Reduce speed to low. Gradually add melted butter; mix until combined. Mix in poppy seeds and lemon zest. Fold in lemon segments and juice. Divide batter among prepared pans.
Bake until a cake tester inserted into centers comes out clean. (If the cakes become too dark while baking, cover with foil.) Transfer to wire racks; let cool in pans slightly. Invert cakes onto racks. Remove parchment; reinvert. Cakes can be wrapped in plastic and stored up to 1 day.
6-INCH: 6 cups batter, 1 hour; 8-INCH: 9 cups batter, 1 hour 30 minutes; 10-INCH: 12 cups batter, 1 hour 30 minutes.
Cut each cake in half horizontally. Spread buttercream about 1/8 inch thick on top of 1 layer; pipe a buttercream dam around edge. Fill with lemon curd. Top with remaining layer. Frost tiers with buttercream. Garnish with candied lemon slices.
6-INCH: 1 cup curd, 1 1/2 cups buttercream (1/4 cup in middle); 8-INCH: 2 cups curd, 2 1/2 cups buttercream (1/2 cup in middle); 10-INCH: 3 cups curd, 4 cups buttercream (1 cup in middle).
Make this batter recipe 2 times for a 3-tiered cake to serve 70. Bake 1 cake for each tier, and cut into 2 layers.