Cup of Yum
1. Preheat oven to 350 degrees F. Butter and line the bottom of a 9-inch round cake pan (ideally with a removable bottom) with parchment paper; lightly butter parchment.
2. Sift hazelnut flour into a mixing bowl. If you have a lot of larger bits leftover, either add some more flour in or pulse the large bits a few times in a food processor to shrink them down, then sift again. You should have a total of 300g after sifting. Whisk in cornstarch, espresso powder and salt; set aside.
3. In the bowl of stand mixer fitted with the whisk attachment, whip egg yolks and 1 cup of granulated sugar on medium-high speed for 5 to 6 minutes or until thick and pale yellow in color.
4. Place 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 barely melted. Ideally the butter should be melted and pourable, but still uniformly light yellow in color and not separated. Let cool slightly.
5. With the mixer running on low, gradually pour butter into mixing bowl with egg yolks, in 3-4 additions, letting it incorporate fully before adding more. Mix in vanilla extract.
6. If you only have one bowl for your stand mixer, transfer the egg yolk mixture to a separate large mixing bowl, then wash the mixer bowl and whisk attachment very well (even a speck of fat left on the bowl will prevent the egg whites from whipping up). Or if you have two bowls (which is ideal), swap them out and just wash the whisk attachment.
7. In a clean mixing bowl with a clean whisk attachment, whip egg whites on medium speed until frothy. Add remaining 1/3 cup sugar in 3 or 4 additions with the mixer running. Once all the sugar has been added, increase the speed to medium high and continue to whip until egg whites hold medium peaks, this could take up to 5 to 10 minutes.
8. Using a wide spatula, fold a little less than half of the whipped egg whites into egg yolk mixture to lighten it, until mostly incorporated. Then fold in half of the sifted hazelnut flour until it is almost-but-not-quite incorporated, followed by remaining egg whites and then remaining hazelnut flour. Be sure you are scraping the bottom and sides of the bowl as you go to ensure the ingredients are evenly mixed, but try not to overmix lest you deflate the air in the batter.
9. Pour batter into prepared cake pan. If your pan has a removable bottom, set it on a baking sheet (just in case).
10. Bake for about 40 minutes or until top is set, edges are light golden brown, and a toothpick inserted in the middle comes out clean.
11. 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). Let cool completely. The cake itself can be made ahead of time and stored covered in the refrigerator overnight if necessary; it can also be wrapped in plastic and frozen for up to a month. Let thaw overnight in the fridge before topping with the ganache.

For Ganache Topping:

Cup of Yum
1. Place chopped chocolate and nutella in a heat-proof bowl or, if you have an immersion blender, the blender canister works really well for this.
2. Warm cream and glucose in a small saucepan over low heat until it just begins to steam (don't let it fully boil).
3. Immediately pour hot cream over chopped chocolate; let sit for 30 seconds, then whisk until the chocolate is fully melted and emulsified, or use an immersion blender to blend until smooth.
4. Transfer to a large mixing bowl (whatever bowl you plan to whip it in ideally); press piece of plastic wrap down on the surface of the ganache and then refrigerate for 4-6 hours or overnight to cool completely.
5. When the ganache is fully chilled, whip with an electric hand mixer for a few minutes until lightened in color and increased in volume; try not to overwhip (the ganache will become very thick with almost a matte finish if overwhipped, still edible it just won't spread as smoothly). Spread or pipe onto cake; top with candied hazelnuts if desired then slice and serve immediately or refrigerate until ready to serve.
6. To store, press a piece of plastic or parchment to the cut surfaces, or store in an airtight container if you have one that fits (but it's not essential). The cake keeps quite well in the refrigerator for up to 5 days (though I recommend not adding the candied hazelnuts until just before serving as they might start to dissolve).