In a medium mixing bowl, cream butter and sugar. Add eggs, egg yolks and vanilla; mix well. Blend in melted chocolate. Combine flour and salt; add to creamed mixture. Stir in pecans. Spoon into six greased 10 oz (300 mL) oven proof custard cups; place on baking sheet. Bake in 350 F (180oC) oven for about 10 minutes. Push one square of chocolate into centre of each brownie until square is at same level as dough. Bake 15-18 minutes longer or until toothpick inserted in cake portion comes out clean. Remove from the oven and let cool for 5 minutes. Loosen the edges with a knife; invert onto dessert plates. Sprinkle with icing sugar, if desired. Serve warm.