Apple Cinnamon Sheet Cake

Ingredients

• 2 cups all-purpose flour
• 1 1/2 teaspoons baking powder
• 1/2 teaspoon baking soda
• 1 teaspoon ground cinnamon
• 1/4 teaspoon ground nutmeg
• 1/4 teaspoon salt
• 1/2 cup unsalted butter, softened
• 1 cup granulated sugar
• 3/4 cup brown sugar, packed
• 2 large eggs
• 1 teaspoon vanilla extract
• 1 cup buttermilk
• 2 cups peeled and diced apples (such as Granny Smith)
• 1/2 cup chopped walnuts (optional)