Hot Chocolate Croquettes

In large metal bowl set over pan of barely simmering water, melt butter and chocolate, stirring until smooth. Remove bowl from pan and whisk in sugar. Whisk in eggs, 1 at a time, then whisk in rum. Add flour and whisk until fully incorporated. Make sure sugar is completely dissolved— the mixture should appear smooth and shiny. Pour mixture into shallow ungreased pan and refrigerate until firm enough to pipe, at least 8 hours. Place corn flakes on large plate. Line 2 large baking sheets with parchment paper. Spoon chocolate mixture into large pastry bag fitted with 1/4-inch tip. Pipe chocolate onto paper in 1 1/2-inch-long and 1/2-inch-wide logs. (If logs are too soft to pick up, return to refrigerator and chill until firm.) Lightly brush each log with beaten egg yolks, roll in corn flakes, and return to baking sheet. Chill until firm enough to pick up, about 30 minutes. In wide, 6-quart deep, heavy pot over high heat, heat 2 inches oil until deep-fat thermometer registers 350 °F. Fry croquettes in batches (returning oil to 350 °F between batches) until golden brown, about 1 to 2 minutes. Transfer to paper towels to drain, and immediately dust with confectioners' sugar. Serve warm.