Brown sugar buttermilk pie

Ingredients

• 9 deep dish pie crust
• 1 cup packed golden brown sugar
• 3 tablespoons. all purpose flour
• 3 large eggs
• 1 egg yolk
• 1 teaspoon . vanilla
• 1 ½ cups buttermilk
• 3 tablespoons. unsalted butter, melted and cooled