For crust: Combine the graham cracker crumbs, brown sugar, cinnamon and salt in a medium bowl. Mix well with your hands. Add the melted butter, stir well with a fork, then rub the ingredients together well with your fingers until thoroughly mixed. Press the mixture evenly in the prepared pan to form a level layer on the bottom with a lip extending about ¼ inch up the sides of the pan. (Do not prebake the crust).
For filling: Combine the butter and chocolate in the top of a double boiler over not-quite-simmering water. When melted, whisk to smooth, then remove the pan from the heat. Scrape the chocolate into a medium bowl and cool to lukewarm. Preheat the oven to 325oF. Whisk the sugar, eggs, and vanilla into the lukewarm chocolate. Stir in the cake flour, mixing until smooth. Stir in the walnuts. Scrape the batter over the crust and smooth with a spoon. Bake on the center oven rack for 35 minutes only - no longer. Cool on a rack. Refrigerate an hour or so before slicing. Serve at room temperature. Makes 16 bars.
I toasted the nuts before baking and used a mixture of walnuts and cashews.
I also chilled the bars overnight which probably improved the flavor and texture.