Preparation

• Heat the oven to 350°F. Butter a 10-inch springform pan and line the bottom with a round of parchment or waxed paper.
• Spread the whole almonds on a baking sheet and heat them in-the oven, shaking the pan every couple of minutes, until lightly toasted and aromatic, 5 to 9 minutes.
• In a small saucepan over medium heat, melt the butter and then continue to cook it for 5 to 7 minutes. Watch it carefully and when you see brown flecks in the bottom of the pan, immediately pour the butter into a medium bowl, scraping the pan with a spatula to get all the flecks. Set aside to let cool slightly.
• In a food processor, pulse the whole almonds and 3/4 cup of the sugar until finely ground. Pour the mixture into a medium bowl and stir in the cake flour, baking powder, and salt. Set aside.
• Stir the rum, vanilla extract, and sour cream into the browned butter.
• Using a stand mixer or a hand mixer, beat the egg whites in a large, clean, dry bowl, beginning on low speed until they start to foam. Increase the speed to medium high and beat until the whites barely hold peaks. Beat in the remaining 3/4 cup sugar in a very slow, steady stream; continue beating the whites until they hold soft peaks.
• Scoop a large dollop of the egg whites into the butter mixture and stir it in. Gently fold half of the almond mixture into the remaining egg whites and then half of the butter mixture into the egg whites. Repeat, ending with the butter mixture.
• Pour the batter into the prepared pan. Smooth the top with a spatula and give the pan a quick spin to even out the batter. Sprinkle with the sliced almonds. Bake in the middle rack of the oven until well risen and golden, 45 to 50 minutes. The center of the cake should spring back when pressed lightly.
• Let the cake cool in the pan on a rack until warm to the touch, about 30-minutes. Run a knife around the inside edge of the pan and carefully flip the cake onto the rack to release it from the pan. Peel off the parchment. Gently flip the cake again onto a serving plate. Serve warm or at room temperature, dusted lightly with confectioners’ sugar.

Make Ahead Tips

You can make this cake ahead and store it in the freezer in a double layer of plastic wrap for up to four weeks. To defrost the cake, unwrap it, set it on a rack, cover it loosely with a towel, and let it sit at room temperature for about 2 hours.