Preheat the oven to 320°F (160°C), static mode.

In a large bowl, melt the butter and dark chocolate together until smooth. Set aside to cool slightly.

In a stand mixer fitted with a whisk attachment, beat the egg yolks, a pinch of salt, and ¼ cup (50 grams) sugar on high speed until the mixture is pale, thick, and fluffy. Gently fold the whipped yolks into the melted chocolate mixture until fully incorporated.

In a clean mixer bowl, whip the egg whites with the remaining ¼ cup (50 grams) sugar until they form stiff, glossy peaks. Using a spatula, carefully fold the whipped whites into the chocolate mixture, working in batches to maintain as much air as possible. The batter should be smooth and uniform but still light.

Generously butter a 9-inch (22 cm) cake ring or springform pan. If using a cake ring, wrap the bottom tightly in aluminum foil to prevent any leaks. Line the base with parchment paper. Pour in the batter and smooth the top.

Bake for 30 to 35 minutes, until the edges are set but the center remains slightly wobbly. The cake will firm up as it cools. Do not overbake.

Let the cake cool completely, then refrigerate for at least 5 hours. Once fully chilled, remove from the mold, slice, and serve.