1/2 cup walnuts (about 1 3/4 ounces), toasted and coarsely chopped
1 1/2 tsp. pure vanilla extract
3/4 cup mashed very ripe bananas (about 2 medium bananas)
1 large egg, plus 1 large egg white
1/2 cup light-brown sugar
1/2 tsp. baking powder
1/2 tsp. baking soda
3/4 tsp. coarse salt
1/4 cup ground golden flaxseed (from about 2 tablespoons whole)
3/4 cup all-purpose flour
1/2 cup whole-wheat flour
2 tbsp. unsalted butter, melted, plus 1 teaspoon unsalted butter, softened, for pan