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
A 9″-diameter springform pan
Preheat oven to 350°F. 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 in melted butter, sour cream, rum, and lemon zest. Add dry ingredients and fold in until batter is smooth; scrape into prepared pan.
Arrange rhubarb in tight rows over batter, trimming to fit. 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.
Cake can be baked 1 day ahead. Store tightly wrapped at room temperature.