In the top pan of a double boiler over simmering water, melt chocolate, stirring occasionally. Remove pan from heat and whisk butter into chocolate until butter is melted and mixture is smooth. Set aside.
In a small, heavy pot over high heat, bring sugar and corn syrup to a boil, swirling pot occasionally to dissolve sugar. Reduce heat to low and simmer for 5 minutes. Remove from heat and let cool for 5 minutes.
In the bowl of a heavy-duty electric mixer with wire whisk attached, beat eggs and yolks at medium-high speed until thick and frothy, 5 to 6 minutes. Reduce speed to medium low and carefully pour hot sugar syrup into beaten eggs in a thin, steady stream, avoiding the beaters. Remove bowl; use a rubber spatula to fold chocolate mixture into egg mixture.
Pour chocolate mixture into prepared pan. Place pan on a cookie sheet. Bake until cake has risen and set, 45 to 50 minutes. Move pan to a cooling rack and let cool completely. Run a thin, sharp knife around cake to loosen; remove from pan. Cover tightly and refrigerate overnight.
Let cake stand until room temperature and dust lightly with confectioners' sugar before serving.