Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla. Combine flour, baking soda and salt; add to creamed mixture alternately with bananas and sour cream, beating well after each addition., Pour into a greased 13x9-in. baking dish. Bake until a toothpick inserted in center comes out clean, 35-40 minutes. Cool completely on a wire rack., For frosting, in a small bowl, beat butter and confectioners' sugar until smooth. Dissolve coffee granules in milk; add to butter mixture and beat until smooth. Spread over cake. Sprinkle with additional instant coffee, if desired.