Preheat the oven to 325 degrees with rack in center. Spray two 9-by-4 1/2-by-2 3/4-inch loaf pans with cooking spray; line with parchment. Arrange geranium leaves, with tops facing down, along bottom and around sides of pan. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, 2 to 3 minutes. Sift together flour and baking powder. With the mixer set on low, add yolks one at a time to butter-and-sugar mixture, alternating with flour mixture.
With the mixer set on medium speed, beat in vanilla, then cream. Batter will be thick. Transfer batter to a large mixing bowl. Set aside.
Place egg whites in a medium nonreactive bowl and, with an electric hand mixer, whip egg whites to stiff peaks. Using a rubber spatula, stir about one-quarter of the egg whites into batter to lighten. Fold in remaining whites.
Carefully spoon batter into prepared pans. (Additional leaves may be arranged on top of batter if desired.) Run a knife blade through batter a few times to remove air bubbles. Bake until a cake tester comes out clean, 80 to 90 minutes. Let cool on wire rack, 1 to 1 1/2 hours, then remove from pans.