Flourless Pistachio Cake with Chocolate Ganache

Preheat oven to 350°F (175°C). Butter and line the bottom of a 9-inch round cake pan (ideally with a removable bottom) with parchment paper; lightly butter parchment.
Sift pistachio flour and almond flour together into a mixing bowl along with cornstarch and salt; set aside.
Rub orange zest into granulated sugar until fragrant and evenly distributed.
In the bowl of a stand mixer fitted with the whisk attachment, whip eggs on medium speed until frothy. Add orange sugar mixture and increase mixer speed to high and whip for 7 to 10 minutes or until eggs are very pale and fall from the beater in solid ribbons that slowly dissolve into the rest of the mixture.
Place cubed butter in a heat-proof measuring cup or container. Microwave at 50% power for 10 seconds, then stir. Repeat, microwaving in 10-second intervals, stirring in between, until butter is just soft enough to flow with no solid chunks but is not fully melted and separated. It should be the approximate consistency of ranch dressing.
Add olive oil to butter and stir to combine.
With the mixer running on low, gradually pour butter and oil mixture in with eggs, a spoonful or so at a time, letting it incorporate fully before adding more. The batter will lose some of its volume. Add pistachio extract.
Add sifted pistachio flour, then mix on low speed for a second or two, or fold in with a wide spatula, scraping the bottom and sides of the bowl as you go, until flour is just incorporated and no dry streaks remain.
Pour batter into prepared cake pan. If your pan has a removable bottom, set it on a baking sheet (just in case).
Bake for 40 to 45 minutes or until the top is no longer fluid, edges are light golden brown, and a toothpick inserted in the middle comes out clean.
Remove from oven and set on a wire rack and let cool. Run a knife around the edge of the cake, then remove sides (or invert to remove it from the pan). If the top is domed, you can press down slightly with a lightly greased spatula or very gently trim off the top to be level once the cake is cooled (the cake is much more fragile than typical layer cakes).
Cut a strip of acetate to fit around the circumference of the cake. Wrap tightly and secure with tape. Optionally place the cake back into the (cleaned) cake pan for additional support. (Note: if you skip the acetate, see the alternate assembly method instructions and treat the ganache more like frosting).
For the Ganache Topping:

Place chopped chocolate in a heat-proof bowl.
Warm cream in a small saucepan over low heat until it just begins to steam (don’t let it fully boil).
Immediately pour hot cream over chopped chocolate; let sit for 30 seconds, then gently stir or whisk, starting in the center and moving in small concentric circles outward, until the chocolate is fully melted and emulsified.
While ganache is still warm and fluid, pour on top of acetate-wrapped cake. If the top is level and the sides tight against the acetate, it should settle into a neat, even layer.
Alternatively, if you don’t have acetate or don’t want to assemble it that way, let your ganache cool at cool room temperature, stirring gently every 10 to 15 minutes, until it firms to a spreadable consistency, then spread or pipe onto completely cooled cake.
Let the ganache cool gradually at room temperature for about an hour, then sprinkle with slivered pistachios if desired. Refrigerate for 2 to 3 hours or overnight until ganache is fully set, then peel off outer layer of acetate before slicing. The cake is best enjoyed at room temperature when the ganache softens and matches the consistency of the cake.
To store, I like to use the same piece of acetate as a ring around the whole cake, then press a piece of plastic or parchment to the cut surfaces. The cake keeps quite well in the refrigerator for up to 5 days.