Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan by buttering it and lining with parchment paper.

Sift together the pistachio flour, almond flour, cornstarch, and fine sea salt in a mixing bowl.

Combine granulated sugar and orange zest in a separate bowl, rubbing them together to release fragrant oils.

Whip the eggs on medium-high speed until frothy, then gradually add the sugar mixture until pale and thick.

Melt the unsalted butter in a small saucepan and add the extra virgin olive oil; drizzle into the egg mixture and fold gently.

Fold the sifted dry ingredients into the wet mixture until just combined.

Pour the batter into the prepared pan and bake for 40-45 minutes, or until a toothpick comes out clean.

Cool the cake in the pan for 10 minutes, then invert onto a wire rack to cool completely.

For the ganache, warm the heavy cream until simmering, then pour over the chopped dark and milk chocolate. Stir until melted and glossy.

Once the cake is cool, pour the ganache over the top, letting it drizzle down the sides. Chill for 30 minutes to set.

Decorate with slivered pistachios and serve at room temperature.