HEAT oven to 325°F. COAT bottom and sides of two 8- or 9-inch round cake pans with cooking spray. LINE bottoms with waxed or parchment paper; SPRAY paper. DUST bottom and sides of pans with flour; TAP out excess.
SIFT flour, baking powder and salt into medium bowl; SET aside.
BEAT eggs in mixer bowl with whisk attachment on high speed until thick, pale lemon-colored and triple in volume, 5 to 10 minutes. Reduce speed to medium. Beating constantly, ADD sugar, 1 to 2 Tbsp. at a time, beating after each addition until sugar is dissolved before adding the next. (RUB a bit of mixture between thumb and forefinger; it should feel completely smooth.) BEAT in vanilla.
ADD apples and nuts to flour mixture; TOSS to mix. SPRINKLE evenly over egg mixture. FOLD gently but thoroughly until no streaks remain. Do not stir. SPREAD in prepared pans.
BAKE in 325°F oven until cakes begin to pull away from sides of pans and wooden pick inserted in center comes out clean, 30 to 40 minutes for 9-inch pans; 35 to 45 minutes for 8-inch pans. COOL on wire racks.
LOOSEN cakes from sides of pans with thin knife. Gently SHAKE cakes out of pans onto racks, taking care not to crack the meringue tops. REMOVE waxed paper carefully; TURN cakes right-side up.
Serve with whipped cream and garnish with apple slices and pecan halves.
Combining heavier mixtures with beaten eggs can knock the air out of them. Add the flour mixture to the beaten eggs, not vice versa. Fold with a light touch rather than stirring. Using a rubber spatula, start with a downward stroke into the bowl, continue across the bottom, up the side and over the top of the mixture. Come up through the center every few strokes and rotate the bowl often as you fold. Fold just until no streaks remain.