Generously butter and lightly flour a 13x9x2-inch baking pan. Arrange geranium flowers or other edible flower petals in a single layer on bottom of prepared pan; set aside. In a medium bowl, combine flour, lemon peel, and salt; set aside.
In a very large mixing bowl, beat eggs, egg yolks, and granulated sugar with an electric mixer on medium to high speed for about 6 minutes or until very thick and pale yellow, scraping bowl frequently.
Gently fold one-third of the flour mixture into the egg mixture. Repeat with remaining flour mixture. Gradually pour one-third of the melted butter over batter, gently folding to combine. Repeat with the remaining butter.
Carefully pour the batter into the prepared pan over the flowers. Bake in a 375 degree F oven for about 30 minutes or until a wooden toothpick inserted into the cake comes out clean and the top is a light golden brown.
Cool cake in the pan on a wire rack for 15 minutes. Loosen side of cake from pan by running a knife around edge of cake. Invert cake onto a large serving platter and cool completely.
To serve, lightly sprinkle top of cake with Rose Geranium Flavored Sugar. Garnish with bouquet of fresh geranium flowers, if you like. Serve with whipped cream, if you like. Makes 12 servings.
Gently wash small rose-scented geranium leaves (or lemon-, mint-, apple-, or nutmeg-scented leaves) or rose petals in water. Drain; place on paper towels and let stand, uncovered, to dry completely. In a clean 1-quart glass jar or bottle, layer the geranium leaves or rose petals with sugar or extra-fine granulated sugar. Cover the jar or bottle tightly with a lid. Let stand at room temperature for at least 24 hours or up to 1 week to absorb oils from geranium leaves or rose petals. To use, sprinkle the flavored sugar over cakes, cookies, fresh fruit, baked custard, or sorbet. Makes about 1 cup.