Heat oven to 325℉/165℃.
Place walnuts on quarter sized sheet pan.
Bake for 5-8 minutes, stirring frequently, until lightly toasted and fragrant. Set aside to cool.
Chop and divide.
Line a muffin tin with paper baking cups, or spray with baking spray. Set aside.
In a medium bowl, smash bananas with vanilla and lemon juice.
With an electric mixer, beat butter and sugar until light and fluffy, about 2-3 minutes.
Add eggs, one at a time, beating well after each addition.
Stir in banana mixture and 1 cup walnuts.
Add flour, baking soda, and salt.
Mix only until blended.
Use a 3 Tablespoon cookie scoop to portion batter, or fill each muffin cup ¾ full.
Top with remaining walnuts.
Bake for 30-35 minutes, or until done.