Preheat your oven to 350°F (175°C). Grease the 9-inch round cake pan with butter and line the bottom with parchment paper for easy removal.
In a double boiler or microwave-safe bowl, combine the chopped chocolate and cubed butter. Melt gently, stirring until smooth and completely combined. Set aside to cool slightly.
In a large mixing bowl, whisk together the egg yolks and sugar until pale and fluffy. This should take about 3-5 minutes. Stir in the yogurt, vanilla extract, and a pinch of salt.
Once the chocolate and butter mixture has cooled slightly, add it to the egg yolk mixture. Stir until fully combined.
In a separate bowl, mix together the almond or hazelnut flour and buckwheat flour. Gradually fold this mixture into the wet ingredients until no dry streaks remain.
In a clean bowl, whip the egg whites until stiff peaks form. This will give your cake a light and airy texture. Gently fold the whipped egg whites into the chocolate batter in three additions, being careful not to deflate the mixture.
Pour the batter into the prepared cake pan and smooth the top. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely. Dust with powdered sugar or serve with a dollop of whipped cream if desired.
