Preheat oven to 425°. In a small heavy saucepan, heat butter and chocolate over low heat until blended, stirring constantly; transfer to a large bowl., Add eggs, egg yolks, sugar, extract and salt to chocolate mixture; mix well. Stir in flour. Pour into four greased 6-oz. custard cups or ramekins., Place custard cups on a baking sheet. Bake 10-12 minutes or until a thermometer reads 160° and edges of cakes are set., Remove from oven; let stand 1 minute. Run a knife around sides of cakes; remove to dessert plates. Dust with confectioners' sugar. Serve immediately.