Preheat your oven to 350F. Butter and line a standard loaf pan with a sheet of parchment paper. The paper is optional, but provides a convenient 'sling' to lift the bread out after baking.
In a large mixing bowl whisk together the flours, sugar, baking powder and salt. Be sure to work out all the lumps in your brown sugar.
Add the Guinness and stir to combine. Mix until everything is incorporated and there is no dry flour left, but don't over mix.
Turn into your prepared pan spread out evenly. Top with the rolled oats. At this point you can bake the bread right away, but I recommend letting it rest on the counter for 20 to 30 minutes for best results.*
Bake for about 35 minutes, until risen and golden. A toothpick inserted in the center should come out without obvious wet batter on it (moist crumbs are ok.)
Let the bread cool a bit before slicing and enjoying. I don't like to slice it until I'm ready to eat it, otherwise it can start to dry out.
The bread will keep, covered in foil or plastic, or in an airtight container, for up to a week. It can be frozen for longer storage.
Notes
*Why let your beer bread rest before baking?
Allowing beer bread to rest before baking can be a great way to enhance its flavor and texture. During the resting period the yeast in the beer will become activated and begin to ferment the sugars in the dough, which will help to create a light and airy texture in the bread. Also, the gluten in the flour will have time to relax and develop, which will result in a better crumb structure.