1. Preheat oven to 375°F. Line bottom of a 9-inch springform pan with parchment or waxed paper. Grease the bottom and sides of the pan with butter. Chop the untoasted walnuts and flour in a food processor fitted with the blade until smooth, set aside.
2. Combine the chocolate and butter in a heatproof glass bowl or measuring cup. Microwave on HIGH 1 minute; stir mixture then microwave for additional 30 seconds. Stir until chocolate is melted; set aside.
3. Beat the egg yolks, ¾ cup sugar and vanilla on high in the bowl of an electric mixer fitted with the whisk attachment, until pale yellow, 5-8 minutes. Stir in chocolate mixture then stir in reserved walnut and flour mixture.
4. Combine the egg whites, cream of tartar and a small amount of the remaining ¼ cup sugar in the bowl of an electric mixture. Whip on high speed with the whisk attachment until the whisk leaves visible paths in the egg whites and the mixture looks light, glossy and fluffy. Beat in the remaining sugar and continue whipping until stiff peaks form, about 1 to 2 minutes
5. Fold egg whites into chocolate mixture in thirds until it is well blended. Gently transfer the batter to the prepared springform pan.
6. Bake until a toothpick or cake tester comes out of the cake clean, 35 to 40 minutes. Cool in pan; the center will sink a little as it cools, don't worry. Run a paring knife around the edge of the pan to make sure the edges are free before unmolding and removing the top part of the pan
7. Use a fine sieve to sprinkle confectioner's sugar over top. Sprinkle with toasted walnuts and raspberries before serving.
Note: This cake has a rich but light texture. Because it contains so little flour, the cake has to be cut carefully with a sharp knife. It can be placed, with the bottom of the pan, on a larger plate so that pieces can be taken neatly.