Cut pound cake into 2 horizontal layers. Set aside. In a med. Bowl, mash thawed blackberries. Add walnuts & raspberry jam to mashed blackberries. Spread layers of pound cake w/ blackberry & raspberry jam mix. Set aside. In mixing bowl, add bananas into whipped topping. Mix well. Immediately top each berry mix layer w/ the banana mix. Stack layers. In a small bowl, combine sour cream, confectioners sugar & vanilla extract. Spread sour cream mixture on top. Cut & serve.
