Big Banana Muffins

Name: Big Banana Muffins
Ingredients: 1 very ripe banana, peeled and lightly mashed (½ cup; 4 ounces; 113.5 grams); 1/4 cup sour cream (about 1/4 cup; 60 grams); 1 large egg (3 tablespoons; 1.75 ounces; 50 grams (weighed without shell)); 1 teaspoon grated lemon zest (2 grams); 3/4 teaspoon pure vanilla extract; 1 cup plus scant 3/4 cup bleached cake flour or bleached all purpose flour (3.5 ounces; 100 grams); 6 tablespoons sugar, preferably turbinado (2.6 ounces; 75 grams); 1/2 teaspoon baking powder (2.4 grams); 1/2 teaspoon baking soda (2.5 grams); 1/4 teasoon salt (1.7 grams); 5 tablespoons unsalted butter, softened (2.5 ounces; 71 grams); 1/2 cup walnuts, toasted and chopped medium fine (1.75 ounces; 50 grams)