1) Preheat the oven to 180C/Gas 4. Coat a 9-inch pie plate with cooking spray.
2) Combine the sugar, buttermilk, biscuit mix, melted butter, vanilla, and eggs in a blender and process until smooth. Pour into prepared pie plate and bake until browned and still a little jiggly (the pie will set as it cools), 30 to 40 minutes.
3) Let cool for 10 minutes before slicing into wedges. If desired, sift confectioners' sugar or cocoa (or a combo) over the wedges before serving.