• prep: 10 min
• cook: 4 hr
• total: 4 hr 10 min

servings:

• 12 tablespoons (1 1/2 sticks) unsalted butter at room temperature
• 1 1/2 cups granulated sugar
• 3 extra-large eggs at room temperature
• 1 1/2 teaspoons pure vanilla extract
• 1 1/4 cups sour cream
• 2 1/2 cups cake flour (not self-rising)
• 2 teaspoons baking powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon kosher salt
• 1/4 cup light brown sugar, packed
• 1/2 cup all-purpose flour
• 1 1/2 teaspoons ground cinnamon
• 1/4 teaspoon kosher salt
• 3 tablespoons cold unsalted butter, cut into pieces
• 3/4 cup chopped walnuts, optional
• 1/2 cup confectioners' sugar
• 2 tablespoons real maple syrup

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