Preheat oven to 325 degrees Fahrenheit and grease an 8-inch springform pan.

Melt the chocolate and butter together using a double boiler or microwave in 30-second increments until smooth.

In a large bowl, whisk egg yolks with 1/4 cup of sugar until pale and thickened.

Slowly fold the melted chocolate mixture and vanilla extract into the egg yolk mixture.

In a separate clean bowl, beat egg whites with salt and cream of tartar until soft peaks form.

Gradually add the remaining 1/4 cup of sugar to the egg whites and continue beating until stiff, glossy peaks form.

Gently fold one-third of the egg whites into the chocolate base to lighten the density.

Fold in the remaining egg whites until no white streaks remain, taking care not to deflate the batter.

Pour the batter into the prepared pan and bake for 35 to 40 minutes until the edges are set but the center remains slightly wobbly.

Cool completely in the pan on a wire rack and dust with cocoa powder before serving.