Chocolate Nut Upside Down Cake

Ingredients

• Caramel
• 3/4 cup packed dark brown sugar
• 2 1/2 ounces unsalted butter
• 3 tablespoons water
• Nuts
• 1 1/4 cups toasted assorted unsalted nuts (toasted, skinned and chopped hazelnuts, slivered almonds, or walnuts are good)
• Cake
• 6 ounces flour
• 1 1/2 ounces unsweetened cocoa powder
• 3/4 teaspoon baking powder
• 1/4 teaspoon baking soda
• 1/4 teaspoon salt
• 5 ounces unsalted butter, at room temperature
• 1 cup sugar
• 1 teaspoon vanilla
• 3 large eggs
• 1/2 cup buttermilk