How to Make It
Heat the oven to 325°F. Butter a 9-inch springform pan with removable sides. Set a heatproof bowl over a pot of barely simmering water. Add the butter and the chocolate to the bowl and heat until melted and smooth, stirring occasionally, about 5 minutes. Remove the butter mixture from the heat. Let cool slightly.
Whisk 3/4 cup of the sugar into the butter mixture until smooth. Whisk in the egg yolks, salt, and vanilla. Fold in the flour.
In a clean bowl, beat the egg whites with an electric mixer on medium until they are light and foamy. Add the remaining 1/4 cup sugar and beat until stiff peaks form, about 3 minutes. Stir about 1/3 of the egg whites into the chocolate batter, then gently fold in the remaining whites.
Transfer the batter to the prepared pan and smooth the top. Sprinkle with the Snickers bars. Bake until a toothpick inserted into the center of the cake comes out with moist crumbs attached, 30 to 35 minutes. Transfer cake to a rack to cool completely. To serve, remove the pan sides and dust with cocoa powder.