Tangerine Upside-Down Cake

Preheat oven to 350 degrees F. Coat an 8- to 9-inch round cake pan with cooking spray; line the bottom with parchment paper and spray the paper.

Heat brown sugar and 2 tablespoons butter in a small pan over medium heat until the butter melts. Spread in the prepared pan, leaving a 1/4-inch border around the edge. Grate 1/2 teaspoon zest from 1 tangerine (or clementine); set aside. Cut a thin slice off the stem end of each fruit, then slice into 1/4-inch rounds. Place a slice in the center of the pan; arrange the remaining slices in slightly overlapping circles out to the edge.

Whisk flour, baking powder, baking soda and salt in a medium bowl.

Beat the remaining 2 tablespoons butter, oil and granulated sugar in a large bowl with an electric mixer until light and fluffy, about 2 minutes. Beat in eggs, one at a time. Mix in vanilla and the zest. With the mixer on low, blend in the flour mixture alternately with buttermilk, making 3 additions of the flour mixture and 2 of the buttermilk and mixing until just combined. Pour the batter into the pan, smoothing the top.

Bake until a toothpick inserted in the center comes out clean, 40 to 50 minutes. Cool for 10 minutes on a wire rack. Run a knife around the edge of the pan and invert the cake onto a cake plate.