In food processor, process 3/4 cup sour cherries for 2 pulses. Transfer the cherries to a large bowl. Add buttermilk, sugar and eggs and beat for about 1 minute. Sift the dry ingredients together. Fold into the cherry mixture and beat until well blended. Preheat oven to 375 degrees and butter a 9 inch springform pan. Pour batter into the pan and bake until a cake tester comes out clean, 40 minutes. Cool the cake completely on a rack. To make the cream, whisk the sour cream and sugar together in a medium bowl for 2 minutes. Add the cherries, walnuts and liqueur and beat until well blended. Remove cake from the pan. Carefully cut cake crosswise into three layers. Set one layer, cut side up, on a platter and brush with 1/4 of the cream. Top with the second layer and repeat procedure. Top with the third layer, cut side up. Spread the remaining cream on top and sides of the cake. Decorate the top with the cherries and walnuts and refrigerate for 8 hours.