Preheat oven to 250~. Chop the chocolate. Put in an ovenproof pan, & melt in the oven, stirring occasionally, until smooth. Remove chocolate, increase oven temp to 325~. Grease a 9" square baking pan. Combine flour, baking powder, & salt. With an electric mixer set on medium-low speed, beat butter & sugar, until light & fluffy. Beat in eggs, one at a time. Slowly, beat in chocolate, then the vanilla. Reduce to low speed, add flour mixture & beat until just mixed. Stir in walnuts. Spread batter into pan. Bake until tested done (toothpick test) about 30-35 minutes. Cool before cutting into squares. Reminder: If using GLASS baking pan, reduce oven heat by 25~.