With a stand or hand mixer set on low speed, beat the butter and sugar in a medium bowl until well blended. Beat in the eggs, one at a time, until fully blended, scraping down the sides and bottom of the bowl as necessary. Beat in the vanilla until blended. Alternately blend the dry ingredients and the melted chocolate into the egg mixture in increments. Using a rubber spatula or wooden spoon, fold in the raisins and walnuts. Stop when the ingredients are just blended. Scrape the batter into the prepared baking pan and spread evenly. Bake about 35 minutes, until a tester inserted in the center comes out clean or with a few crumbs clinging to it. Transfer to a wire rack and let the brownies cool completely, then lift from the pan using the overhanging parchment. Cut into squares.