CITRUS UPSIDE-DOWN CAKE

Steps:

• 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.