Preheat oven to 350°. Generously butter and flour (or use very finely ground walnuts) a 9-inch springform pan. Wrap the bottom of the pan with foil to catch any butter that melts out in the oven.
In a food processor, finely grind the walnuts. This will likely take only 8 -10 pulses. Stop just as they begin to clump. (Any further and you'll make walnut batter, not quite what we are after). Set the ground walnuts aside.
Seperate the eggs, placing the yolks in the bowl of a stand mixer with the whip attachment. Beat the yolks with the sugar and salt 6-8 minutes until the mixture is fluffy and pale yellow. (You'll be glad you let your eggs come to room temperature for this-- the sugar more readily dissolves in eggs that are not cold.)
If you are making a plain flourless walnut cake, move on to step five. If you are making either a walnut-spice cake or a coffee flavored cake, add the spice mix or the finely ground coffee beans now and mix in thoroughly.
Remove the mixing bowl from the stand mixer and with a silicone or rubber spatula, fold the ground walnuts into the yolk mixture.
In a separate clean bowl free of any oils or grease, whip the egg whites, cream of tartar, and vanilla to medium peaks. (The cream of tartar helps stabilize the whipped egg whites.) Gently fold about a third of the egg whites into the walnut mixture. Then, whip the remaining egg whites once again until they just reach firm peaks. Fold them gently into the walnut mixture in two batches, folding until no more white streaks remain.
Place the cake batter into the prepared pan. Place the pan on a baking sheet. Bake for 40-50 minutes, or until the cake is puffed, set (not jiggly) in the middle, and a cake tester (I use a bamboo skewer for this) inserted in the middle comes out clean.
Allow the cake to cool in the pan about 20 minutes before removing the springform ring. The cake will have sunk in the center and formed charming cracks and crags, perfectly normal for this rustic meringue-style cake.
Decorate with seasonal fruits, a dusting a powdered sugar, or the coffee glaze below. Seve with whipped cream.