Step 1Place a rack in the center of the oven and heat the oven to 350 degrees. Butter a 9- or 10-inch-diameter springform pan. Fit the bottom with a circle of parchment paper and butter the paper. Set the pan aside.
Step 2Place the crumbs, almonds, orange zest, baking powder and sugar in a food processor. Process until the almonds are as fine as sugar.
Step 3Add the butter, egg yolks, lemon juice, marmalade and salt to taste. Process the mixture until it's very smooth, about 1 minute, stopping as necessary to scrape down the sides of the work bowl. Transfer the batter to a bowl.
Step 4Beat the egg whites to soft peaks-they should hold their shape but still be moist. Stir 1/4 of the egg whites into the batter, then fold in the remaining whites. Spoon the batter into the prepared springform pan and spread the top to smooth it evenly.
Step 5Bake the cake until it's browned on top and a toothpick inserted in the center comes out clean, about 30 to 35 minutes. It's OK if the cake is slightly sunken in the center. Cool the cake completely on a wire rack before removing it from the pan. (The cake can be stored overnight at room temperature, covered, or it can be frozen up to 1 month, covered airtight. If frozen, bring it to room temperature while it's still wrapped.)
Step 6To serve, transfer the cake to a platter and gently press down on the edges of the cake to make them level with the center. Shake the powdered sugar through a strainer over the top of the cake. Cut it into wedges and serve.