Preheat your oven to 350°F [180°C]. Use your hands to butter the bottom and sides of an 8-in [20-cm] cake pan, then line the bottom with a circle of parchment paper. For good measure, butter the parchment paper. Set the pan aside.
In a medium bowl, whisk together the flour, ground nuts, baking powder, and salt.
In a large bowl, whisk the eggs until the whites and yolks are fully combined. Add the olive oil and granulated sugar and whisk until the sugar is dissolved (test by rubbing some of the mixture between two fingers). Whisk in the vanilla, orange zest, and orange juice. Whisk in the flour mixture.
Pour the batter into the prepared pan, being sure to use a rubber spatula to get it all out of the bowl. Hold the pan just a little bit above the counter and then drop it on the counter to eliminate any air bubbles.
Bake until the cake is beautifully golden brown and a toothpick inserted in the center comes out clean, 25 to 30 minutes. Transfer the cake, still in its pan, to a wire rack and let it cool completely. Once cool, use a dinner knife to loosen the edges of the cake from the pan and invert it onto your work surface (you might need to give the pan a little whack). Peel off and discard the parchment. Invert the cake one more time onto a serving platter so the flat side is down and the domed side is up.
Just before serving, dust the cake with powdered sugar.
Note: Any nut works well in this cake. To make ground nuts, just put whichever type of nut you’d like (I’ve made this cake successfully with walnuts, almonds, hazelnuts, and pistachios, and am sure pecans and pine nuts would also be great) in the food processor and blitz until they’re as fine as cornmeal. Or use a mortar and pestle. Or purchase ground nuts, which often go by the name “meal” or “flour” as in “almond meal” or “almond flour.” For a nut-free cake, simply omit the ground nuts and add an additional ½ cup [60 g] flour.
Feel free to substitute any citrus in place of the orange. Clementine, tangerine, blood orange, and grapefruit zest all work very well.
For a lemon-poppy seed cake, use lemon zest and juice instead of orange and add 1 Tbsp poppy seeds to the batter.
If you’re using ground almonds, add 1/2 tsp almond extract to the batter for a more intense almond flavor.
Just before you put the cake in the oven, dot the top with 1/4 cup [80 g] raspberry jam and use a fork or the tip of a paring knife to swirl in the jam.