How to Make It
Preheat oven to 350°F. Combine 1 1/2 cups sugar, butter, 1/2 cup almond paste, baking powder, salt, and lemon zest in bowl of a food processor. Process until light, creamy, and smooth, about 45 seconds. With processor running, add eggs, 1 at a time, incorporating fully after each addition (mixture should look thick and fluffy). Add sour cream, and process until fully incorporated, about 30 seconds, stopping to scrape bowl as needed.
Whisk together cake flour and all-purpose flour in a large bowl. Add sugar mixture to flour mixture; whisk just until all dry ingredients are incorporated. Using a rubber spatula, fold batter to thoroughly combine ingredients.
Toss together plums and lemon juice in a medium bowl; let stand 5 minutes. Line a 13- x 9-inch baking pan with parchment paper; lightly grease with cooking spray. Spread batter in prepared pan, smoothing top with an offset spatula. Sprinkle remaining 1/2 cup crumbled almond paste over batter. Arrange plums over top, and sprinkle with almonds and remaining 2 tablespoons sugar.
Bake cake in preheated oven until lightly browned and a wooden pick inserted in center comes out clean, 45 to 50 minutes. Transfer cake in pan to a wire rack, and let cool completely, about 2 hours
Almond paste can be found in the baking aisle of most supermarkets.