Maple-Walnut Layer Cake With Easy Maple Frosting

Ingredients

• 1-1/2 cups walnut halves, plus extra for garnish
• 1 cup (2 sticks) unsalted butter, softened, plus extra for pans
• 3/4 cup granulated sugar
• 3/4 cup firmly packed light-brown sugar
• 3 large eggs
• 3 egg yolks
• 1 teaspoon vanilla extract
• 3 cups cake flour, plus extra for pans
• 1-1/2 teaspoons baking powder
• 1 teaspoon kosher or sea salt
• 3/4 teaspoon baking soda
• 1 cup maple syrup
• 1/2 cup milk
• 1/2 cup sour cream