In a large mixing bowl, combine the flours, sugar, soda, and salt. Cut in the butter. Stir in the malt, buttermilk, and Guinness and mix well. Pour into prepared pan and sprinkle with 1 to 2 tablespoons coarse whole wheat flour. Bake for 30 minutes, then reduce heat to 400 F and bake 15 to 30 minutes longer.
Bread is done when a skewer inserted into the center comes out clean.
Turn off the oven and cool with the door open for 30 minutes. Transfer to a wire rack to cool completely. All rights reserved.