Preheat oven to 350 degrees. Prepare 2 9" pans with paper, nonstick spray and flour.
Beat butter and sugar at high speed until fluffy, minimum five minutes. ( I leave mixer going until I have finished the next three steps.).
In separate bowl, whisk together cake flour, baking powder and salt.
Meanwhile, separate 4 eggs. Just in case, separate each egg into small bowl one at a time, then pour the egg white into larger bowl of egg whites.
Now separate 6 more eggs, putting these egg yolks into a new separate bowl. The egg whites can be combined with the egg whites from the last step.
Add the 4 eggs and 4 egg yolks, one at a time, beating well after each addition. Beat in the vanilla.
In separate bowl, whisk together cake flour, baking powder and salt. Beat in the flour mixture at low speed in 3 batches, alternating with the milk.
Pour batter into prepared pans and bake for 35 - 40 minutes. Let the cakes cool for 10 minutes, then unmold them.
MAKE THE CURD.
Fill a large bowl halfway up with water and ice to be used for the water bath later. Bring a pot of water to simmer over med-low heat.
Meanwhile, in pyrex bowl, whisk together six egg yolks, sugar, lemon juice and salt. Place bowl with egg yolk mixture over simmering water, making sure bowl doesn't touch the bottom of the pan.
Continue to whisk vigorously until curd thickens, doubles in volume and deepens in color. It should take about ten minutes. DO NOT LET IT BOIL.
Set the saucepan in the water bath, adding the butter a couple of pieces at a time and whisking continually until butter is melted. Cover and refrigerate.
MAKE THE MERINGUE FROSTING.
In a saucepan, bring 1 inch of water to a boil.
In the clean bowl of a standing mixer, whisk the sugar, egg whites and lemon juice.
Set the bowl of egg whites over the saucepan and cook, whisking constantly, until the sugar dissolves, about 6 minutes.
Return the bowl to the mixer and beat the meringue at medium speed for 10 minutes, or until cooled to room temperature.
Continue beating at high speed until the meringue is thick, shiny and billowy, about 15 minutes longer.
ASSEMBLE THE CAKE.
Using a serrated knife, trim the cake tops to make them level. Halve each cake horizontally to end up with four layers. The layers must be cut level. (I use a cake cutting gizmo to do this.).
Assemble the cake with the lemon curd between layers and frosting sides and top with meringue frosting.
NOTE: I often put a very thin layer of thinned butter cream frosting to "cement" the cake crumbs, letting that frosting harden before adding the meringue frosting. To make this cream about 2-3 T butter with enough powdered sugar to make it thick, then add milk until it is thinned but will still stick to the edges of the cake.