Preheat the oven to 325 degrees. Butter the cake pans (see below for pan sizes), and line each with a parchment-paper circle cut to size. Butter and flour the parchment; set the pans aside. Sift together the flour and baking powder 3 times; set aside.
Beat the butter and sugar together in a mixer bowl until fluffy. Add egg yolks one at a time, beating until incorporated after each. Add the flour mixture to the egg mixture in 3 additions, alternating with milk, starting and ending with flour.
In a clean mixer bowl, whisk the egg whites with the salt until stiff but not dry. Mix a quarter of the whites into the batter, and then fold in the remaining whites.
Pour the batter into the pans. Bake until the tops are golden brown and a tester inserted into the centers comes out clean. Cool the cakes in the pans for 20 minutes. Run a knife around the outsides of the cakes. Turn out the cakes onto wire racks, remove the papers, and reinvert the cakes to cool, top side up. Wrap the cooled cakes in plastic; refrigerate at least overnight before using.
Approximate batter amounts and baking times for 3-inch-deep round pans: 6-inch: 3 1/2 cups, 1 hour; 8-inch: 6 1/2 cups, 1 hour 10 minutes; 10-inch: 9 1/2 cups, 1 hour 15 minutes; 12-inch: 15 cups, 1 hour 35 minutes.
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