Cooking spray
unsalted butter
all-purpose flour
baking soda
very ripe large bananas
large egg
packed light brown sugar
Greek yogurt or sour cream
vanilla extract
kosher salt
fresh or frozen blueberries, divided (about 5 1/2 ounces, do not thaw)
Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat a 8 1/2x5-inch loaf pan with cooking spray; set aside.
Microwave 4 tablespoons unsalted butter in a small microwave-safe bowl in 20-second increments until melted, 40 to 50 seconds total. (Alternatively, melt in a small saucepan over medium heat.) Set aside to cool until just warm.
Place 1 1/2 cups all-purpose flour and 1 teaspoon baking soda in a medium bowl and whisk to combine.