Make the sponge cake the day before or morning you will serve cake.
Preheat oven to 350°F Butter two 9-inch layer cake pans and line the bottoms with wax paper.
Sift the flour, salt and baking powder together and set aside. Beat egg yolks in a large bowl with and electric mixer on high for 5 minutes. With mixer running, gradually add 3/4 cup sugar and beat until thick and light yellow, about 5 minutes. Beat in vanilla and lemon extract. Sift flour mixture over batter and stir in by hand until no white remains. Then blend in butter stirring until it no longer looks separated. In a clean bowl, beat egg whites with cream of tartar on high until frothy. Gradually add remaining 1/4 cup sugar and beat until stiff peaks form. Stir about 1 cup of whites into batter, then gently fold remaining whites inches Divide batter evenly between pans and bake until layers spring back when lightly touched, about 25 minutes. Let cool on rack 30 minutes, then turn out of pans. Peel of paper and let cool completely (at least 4 hours).
Prepare the Lemon Filling 2 hours before serving.
Mix sugar, cornstarch and salt together in large saucepan. Gradually pour in the water, whisking to prevent lumps. Cook over medium heat until mixture thickens. Remove from heat. Meanwhile beat yolks with electric mixer on high, until thick and light. Mix a little hot starch mixture into eggs to warm, then pour eggs into pan. Cook over low heat, whisking, just until hot and edges are just starting to cook. Be very careful not to overcook or it will separate. Remove from heat and whisk in juice, rind, butter, vanilla and lemon extract. Refrigerate until cool to touch, but not chilled and set - should still be spreadable.
Sift sugar and salt together and set aside. Cream butter and margarine together in large bowl with mixer on high for 3 minutes. With mixer still running, beat in corn syrup and vanilla. Reduce mixer to low and beat in sugar in two additions. Blend in cream until spreading consistency is reached, using a little extra if necessary.
Set 1 cup aside in a pastry bag fitted with a medium star tip.
Split each cake layer to make 4 layers. Spread first layer with about 1/5 frosting, then 1/4 lemon filling. Repeat with second and third layers. Top with last layer. Frost sides of cake, then pat with coconut, reserving about 1/4 cup. Frost top of cake with remaining frosting and then spread with remaining filling. Sprinkle with 1/4 cup coconut. Using pastry bag, pipe scrolls of frosting along top edge of cake and large rosette in center.