What makes these muffins so moist is to choose very ripe bananas. If the bananas are mottled and overripe, then that's even better because they'll add flavor, sweetness and moistness to the muffins. Don't assume that brusied looking bananas are ripe because they can still be firm on the inside. For fuller muffins with a big muffin top, divide the mixture into 11 muffin cups instead of 12.
In bowl, whisk together by hand the flour, baking powder, and salt until well combined, about 20 seconds. Set aside.
In another large bowl, cream together the butter and sugar until light and fluffy, about 2 minutes. Beat in the eggs, vanilla and bananas until they are well broken up.
By hand, fold in the flour mixture until just combined. Fold in the nuts, reserving a handful to top the muffins with. Don't overmix which can result in tough muffins. Fill the muffin cups with the batter. Top with the reserved nuts.