Preheat oven to 220 degrees C. Grease a 23 x 30cm baking tin.
Mix together the oats, flour, sugar, bicarbonate of soda, baking powder and salt in a large bowl. In a separate large bowl, stir together the butter, vanilla, buttermilk and Guinness. Pour the flour mixture into the beer mixture and gently stir until well blended. Pour mixture into the prepared baking tin, and sprinkle with additional oats if desired.
Bake in preheated oven for 30 minutes, then turn the temperature down to 200 C and bake for an additional 30 minutes. Turn the oven off, open the door, and allow to cool for 30 minutes in the oven before turning out onto a wire rack.