Preheat oven to 350° F. Butter and flour a 9" springform pan, and set aside.

In a medium bowl, whisk together baking powder, salt, and 1 cup all-purpose flour.

In a large bowl, vigorously whisk eggs, egg yolk, and 1½ cups sugar in a large bowl until very pale and thick, about 1 minute.

In a small bowl, whisk melted butter, sour cream, and lemon zest.

Whisk butter mixture into egg mixture, just to combine. Then add dry ingredients and fold with a spatula until batter is smooth, and scrape into prepared pan. Place in refrigerator to chill for 10 minutes, to let the batter set.

Arrange rhubarb over batter however you like, trimming as needed. To create the patchwork pattern you see in my photos, refer to the tutorial photos in the post. Do not press the rhubarb into the batter, let it rest gently on the surface.

Sprinkle with 1 tablespoon of sugar and bake until cake is golden on top and browned around the edges, about 45-55 minutes or so. Transfer pan to a wire rack and let cake cool in pan 10 minutes. Then slide a knife around sides of cake to loosen and unmold. Let cake cool completely. Slice into individual pieces and serve with whipped cream if desired.