Preheat oven to 350F. Grease 9-inch loaf pan; dust with flour. Combine sour cream and baking soda in large bowl; let stand for 5 minutes. Mix butter, sugar, and eggs together in separate bowl; stir into sour cream mixture. Add mashed banana; mix well. Gradually add vanilla and flour; mix well after each addition. Pour batter into prepared loaf pan; bake for 1 hour. Cool in pan on wire rack for 10 minutes, then turn out onto rack; cool completely before frosting.

To make frosting, cream butter and sugar together well in mixing bowl. Stir in milk and lemon juice, beating well until frosting is of spreading consistency. Frost sides and top of cooled cake.