Citrus Upside-Down Cake

Preheat the oven to 325°F with a rack in the center position.  

To make the topping: Brush the bottom and sides of a 9" round cake pan with the melted butter. In a small bowl. combine the sugars. Zest the lemon into the bowl, then use your fingers to rub the zest into the sugar until the mixture is fragrant and moist. Sprinkle all of the lemon sugar evenly over the bottom of the pan, then shake the pan so that the sugar sticks to the sides. (Leave any loose sugar in the bottom of the pan.) Juice the lemon and set aside. 

Finely grate the zest of 1/2 an orange and reserve for the batter. Use your hands to peel the fruits (you can leave the peel on one smaller tender-skinned fruit, such as a clementine, if desired). Slice the fruit crosswise into 1/4"-thick rounds and remove any seeds. Arrange the sliced fruit over the sugar mixture in a snug, slightly overlapping layer (for the most striking appearance, place smaller circles down first); halve some circles if necessary. Any leftover citrus that doesn’t fit is a baker’s snack! 

To make the upside-down cake: In a medium bowl, whisk together the flours, 1/4 cup (50g) of the sugar, baking powder, salt, and baking soda.  

In a separate medium bowl, whisk the eggs, remaining 1/2 cup (100g) sugar, and reserved orange zest until very pale in color and fluffy, about 2 minutes. Whisk in the vanilla and orange blossom water or almond extract, then gradually whisk in the olive oil until completely emulsified. Whisk in 2 tablespoons (28g) of the reserved lemon juice from step 1. Whisk in the flour mixture until just combined.  

Scrape the batter into the pan and gently spread to cover the fruit. (A small offset spatula is a helpful tool here.) 

Bake the citrus upside-side cake for 35 to 40 minutes, until the top is golden brown and springs back when lightly touched. 

Remove the cake from the oven and let it cool in the pan on a wire rack for 10 minutes. Run a paring knife or small offset spatula around the edges of the pan to loosen it, then invert the cake onto a wire cooling rack lined with parchment, which will prevent the cake from sticking. Use the parchment to ease the cake onto a serving plate, if desired. Serve the cake warm or at room temperature. 

Serving information: The citrus upside-down cake is best enjoyed the day it’s baked. Store leftovers loosely covered for 1 day at room temperature.