Sour Cream Coffee Cake

INGREDIENTS

• ¼ cup light brown sugar, packed
• 12 tablespoons (1½ sticks) unsalted butter at room temperature
• 1½ cups granulated sugar
• 3 extra-large eggs at room temperature
• 1½ teaspoons pure vanilla extract
• 1¼ cups sour cream
• 2½ cups cake flour (not self-rising)
• 2 teaspoons baking powder
• ½ teaspoon baking soda
• ½ teaspoon salt
• :
• ½ cup all-purpose flour
• 1½ teaspoons ground cinnamon
• ¼ teaspoon salt
• 3 tablespoons cold unsalted butter, cut into pieces
• ¾ cup chopped walnuts, optional
• ½ cup confectioners' sugar
• 2 tablespoons real maple syrup