Grandma's Banana Cookies

Ingredients (15 items)

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• 4 cups all-purpose flour
• 2 teaspoons baking powder
• 1 teaspoon salt
• 1 cup mashed ripe banana
• ½ cup sour cream
• 1 teaspoon vanilla extract
• 1 cup margarine
• 1 cup white sugar
• 2 large eggs
• 1 teaspoon baking soda
• 1 cup chopped walnuts
• ½ cup unsalted butter
• 1 cup brown sugar
• ¼ cup milk, or more as needed
• 2 ½ cups powdered sugar, or to taste