Ingredients
1 stick unsalted butter, softened, plus more for pan
1 1/4 cups unbleached all-purpose flour
1 teaspoon baking powder
1/4 teaspoon baking soda
1 teaspoon kosher salt (we use Diamond Crystal)
1 cup sugar
1 1/2 teaspoons finely grated tangerine or orange zest
2 large eggs, room temperature
1 teaspoon pure vanilla extract
1/2 cup sour cream, room temperature
3 to 4 tangerines or oranges, such as blood oranges, Cara Cara, or mandarins, or a combination
4 tablespoons unsalted butter, melted
1/2 cup sugar
1 tablespoon dark rum, such as Myers's (optional)
Preparation
Cake: Preheat oven to 350°F. Brush an 8-inch square pan with 2-inch-high sides with butter. Whisk together flour, baking powder, baking soda, and salt.
In a mixer fitted with the paddle attachment, beat butter with sugar and zest on medium speed until light and fluffy, 2 to 3 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla and sour cream, scraping down sides of bowl as needed. Reduce speed to low and gradually add flour mixture.
Topping: With a sharp knife, slice ends off each citrus. Following curve of fruit, cut away peel, removing as much white pith as possible. Slice fruit crosswise into 1/4-inch-thick rounds, and remove any seeds. In a small bowl, stir together butter, sugar, and rum. Spread mixture over bottom of prepared pan. Arrange sliced citrus over it in a snug, slightly overlapping even layer (cut some to fit into corners).
Spread batter over citrus. Bake until cake is golden and puffed and a tester inserted in center comes out clean, about 55 minutes. Transfer pan to a wire rack and let cool 30 minutes. Invert onto a cake plate; serve the same day, warm or room temperature.
