Preheat Oven to 325~. Grease a 9" square baking pan. In a large saucepan, melt the butter over low heat, & stir in the sugar until smooth. Remove from heat, & cool completely. Chop the chocolate. Combine the flour, baking powder, & salt. Beat in eggs, one at a time, into melted butter mixture. Beat in vanilla. Stir in flour mixture, then chocolate chunks. Spread the batter in the pan. Bake until edges start to come away from sides of the pan. about 50-55 min. Cool completely before cutting into squares. Reminder: If using a GLASS baking pan, reduce oven heat by 25~.