Preheat the oven to 350. Butter a 9-inch springform pan and place it on a baking tray.
Melt the butter, vanilla and espresso (or espresso powder) in a saucepan then pour it over the chocolate. Let sit for 1-2 minutes then stir with a whisk until the mixture is smooth.
In a separate bowl whisk together the egg yolks and 1/2c sugar. Whisk until the mixture becomes thick and pale then fold in the hazelnut meal followed but the melted chocolate mixture.
In a stand mixer beat the egg whites and remaining 1/2c of sugar on high until glossy stiff peaks form about 5 minutes. Gently fold the whipped egg whites into the chocolate/yolk mixture. Transfer the batter to the prepared pan and bake until puffed slightly and set along edges but still slightly wobbly in center, 35 to 40 minutes. LEt cool complety then slide a paring knife along the sides of the cake pan to release any bits that may be stuck then release the sides. Slice and serve with warm espresso!
Hazelnut flour can be hard to find (and expensive!) feel free to substitute almond flour instead.