This black walnut banana bread takes the friendship between walnuts and quick breads up a notch by including the nuts in the batter and in the buttery streusel topping. Because it’s not just enough to sprinkle walnuts on top—for it to really be the best possible slice, the flavor of walnuts should be in every bite. A buttery, crunchy streusel provides textural contrast to the moist bread. The simple buttermilk glaze offers a little sweet tang.