apple upside-down cake (walnuts optional)

Ingredients

• 6 tablespoons unsalted butter
• 2 medium to large granny smith apples (peeled, cored and sliced thin)
• 2/3 cup light brown sugar (packed)
• 3 tablespoons pure maple syrup
• 2 pinches kosher salt
• 1/4 cup walnuts, toasted, cooled and finely chopped
• 1 1/2 cups all purpose flour
• 2 teaspoons baking powder
• 1 teaspoon ground cinnamon
• 1/2 teaspoon kosher salt
• 1/2 cup (one stick) unsalted butter, room temp
• 3/4 cup sugar
• 2 large eggs
• 1 teaspoon vanilla extract
• 1/2 cup milk