Step 1Heat the oven to 350 degrees. Butter a 9-inch-square baking pan.
Step 2Combine the chocolate and butter in the top of a double boiler set over, but not touching, simmering water and stir until melted and smooth. Remove from the heat and cool slightly.
Step 3Whisk the egg yolks with the sugar and whisk into the chocolate mixture. Whisk in the vanilla. In a clean bowl, beat the egg whites with salt just until soft peaks form. Carefully fold into the chocolate mixture. Pour into the prepared pan. Bake until set but not dry, 30 to 35 minutes.
Step 4Cool slightly, then cut into squares and serve. Serve with the bitter orange sauce drizzled over top.