Preheat the Oven - Set the oven rack to the middle position and the heat to 350ºF (177ºC). Line a 9x5" metal loaf pan with a 9" wide piece of parchment paper that overhangs about 1 inch. The sling will make handling the bread easier. Grease the bottoms and sides of the paper and pan with cooking spray.
Mix the Dry Ingredients - In a large bowl, whisk the whole wheat flour, oats, baking soda, and salt.
Incorporate the Butter - Using your fingers, break the butter into very small pieces until the flour mixture resembles cornmeal. Stir in the brown sugar. Make a well in the center of the bowl.
Add the Wet Ingredients - In a medium bowl or large measuring cup, whisk the buttermilk, Guinness, and molasses. Add the mixture to the center of the flour and stir until a batter forms. Do not overmix!
Bake - Transfer the batter to the prepared loaf pan, spreading it into an even layer. If desired, sprinkle oats on top. Bake until the internal temperature reaches 190ºF (88ºC) on an instant-read thermometer or a toothpick inserted in the center comes out clean, about 50 to 55 minutes.
Serve - Lift the loaf out of the pan and transfer it to a wire rack to cool. Slice and serve warm.
Notes
Baking Soda Substitute: Use 1 tablespoon of baking powder.
Molasses Substitute: Use maple syrup or honey. The bread will not be as dark brown.
Substituting Guinness: Use all buttermilk (the bread won't be as dark), or use root beer instead.
Smaller Loaves: Bake two mini loaf pans (about 5 by 3 inches) on a baking sheet. Check at 35 minutes for doneness, adding more time as needed. Loaves are ready when they reach 190°F (88 °C).
Storing: Cool completely, then wrap in foil and place in a plastic bag. Store at room temperature for up to 5 days. Freeze for up to 3 months. Defrost the loaf before using it.
Reheating: Toast room temperature or frozen slices of bread.