This cake is designed to be mixed by hand. Using an electric mixer works more air into the eggs, which lightens the batter and makes it less dense, so the fruit is more prone to sinking. Also, make sure you leave the stalks whole, as chopped rhubarb is more likely to be swallowed up by the batter and sink to the bottom.
4 Tbsp. melted unsalted butter, cooled, plus more room-temperature for pan
1 cup all-purpose flour, plus more for pan
3/4 tsp. baking powder
1/2 tsp. kosher salt
2 large eggs
1 large egg yolk
1 1/2 cups sugar, plus more for sprinkling
1/4 cup sour cream
2 Tbsp. dark rum
2 tsp. finely grated lemon zest
13 oz. rhubarb stalks, halved lengthwise if thick
A 9″-diameter springform pan
Preheat oven to 350. Butter and flour pan. Whisk baking powder, salt, and 1 cup all-purpose flour in a medium bowl. Whisk eggs, egg yolk, and 1 1/2 cups sugar in a large bowl until very pale and thick, about 1 minute. Whisk melted butter, sour cream, rum, and lemon zest in a small bowl. Whisk butter mixture into egg mixture just to combine. Add dry ingredients and fold in until batter is smooth; scrape into prepared pan. Chill 10 minutes to let batter set.
Arrange rhubarb over batter however you like, trimming as needed. Don’t press fruit into batter—just place over top and let it rest on the surface. Sprinkle with more sugar and bake until cake is golden on top and browned around the sides, 45–55 minutes. Transfer pan to a wire rack and let cake cool in pan 10 minutes. Slide a knife around sides of cake to loosen and unmold. Slide directly onto rack and let cool completely.
– Bon Appétit