HEAT oven to 350°F.
CHOP 20 wafers to form coarse crumbs; reserve for later use. Finely crush remaining wafers; place in medium bowl. Add flour, baking powder and baking soda; mix well. Set aside. Beat 1/4 cup (4 Tbsp.) butter and granulated sugar in large bowl with mixer until well blended. Add vanilla; mix well. Beat in eggs, 1 at a time. Add flour mixture alternately with sour cream, beating well after each addition.
MICROWAVE remaining butter in medium microwaveable bowl on HIGH 30 sec. or until melted. Add reserved chopped wafers along with nuts and cinnamon; mix well. Pour half the cake batter into 12-cup fluted tube pan or 10-inch tube pan sprayed with cooking spray; top with half the nut mixture. Repeat layers.
BAKE 40 to 45 min. or until toothpick inserted near center comes out clean. Cool in pan 10 min. Loosen cake from sides of pan with knife. Invert cake onto wire rack; gently remove pan. Cool cake completely. Place cake, streusel-side up, on platter.
MIX powdered sugar and water until well blended; drizzle over cake.