2.To make lemon curd; whisk eggs, egg yolks and sugar in a medium saucepan until smooth. Chop butter into small cubes; add butter, rind and juice to pan. Whisk over low heat until curd thickens. Remove from heat; strain into a medium bowl. Cover surface with plastic wrap; cool.
3.To make lemon syrup; stir sugar and the water in a small saucepan over medium heat, without boiling, until sugar dissolves. Bring to the boil; boil, uncovered, without stirring, about 3 minutes or until syrup is thickened. Remove from heat; stir in liqueur. Cool.
4.Preheat oven to 180°C/350°F. Grease two deep 22cm (9-inch) round cake pans; line bases and sides with baking paper.
5.Beat butter and sugar in a large bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time until combined. Stir in sifted flours, crème fraîche and juice, in two batches.
6.Divide mixture evenly between pans; smooth surface. Bake about 35 minutes. Stand cakes in pans 5 minutes before turning, top-side up, onto wire racks to cool.
7.Using a serrated knife, trim tops of cakes so they are level. Split each cake into three layers. Place one layer on a serving plate; brush with a little of the syrup and spread with ½ cup lemon curd. Repeat layering with remaining cake layers, syrup and curd, finishing with a cake layer.
8.To make fluffy frosting; combine sugar and the water in a small saucepan; stir with a wooden spoon over high heat, without boiling, until sugar dissolves. Bring to the boil; boil, uncovered, without stirring, about 5 minutes or until the syrup thickens slightly and reaches 114°C/240°F on a sugar thermometer (this is known as ‘soft ball stage’). While the syrup is boiling (after about 4 minutes), beat the egg whites in a small bowl with an electric mixer until stiff; keep beating until the syrup reaches the correct temperature. When the syrup is ready, allow the bubbles to subside then pour a very thin stream into the egg whites with mixer operating on medium speed. Continue beating and adding syrup until all the syrup is used. Continue to beat until the frosting stands in stiff peaks, about 10 minutes (frosting should be barely warm).
9.Spread fluffy frosting over top and sides of cake.
If you don’t have a sugar thermometer when making the fluffy frosting, you can test whether the sugar has reached the correct temperature by dropping a teaspoon of the mixture into a glass of cold water the sugar will form a clear- coloured soft, flexible ball. Remove the sugar ball from the water and press it gently between your thumb and finger and it should flatten easily. You may have to do this test a few times until the soft ball stage is reached. The fluffy frosting should be used immediately, as it sets quite quickly as it cools down and will lose its glossiness. Limoncello is made from the peel only of fragrant lemons. The peel is soaked in a good-quality clear alcohol then diluted with sugar and water.