Preheat oven to 350°F or 325° F convection.
Oil and lightly flour with gfJules Flour one 8.5 x 4.4 loaf pan AND one mini loaf pan or two muffin cups; OR use one larger 9 inch loaf pan.
Cream butter and sugar, whipping until light and fluffy. Add eggs and blend until integrated. If making non-dairy buttermilk, add 1 tablespoon lemon juice to a liquid measuring cup, then add enough non-dairy milk to total 1/2 cup. Allow the mixture to stand while mashing and measuring the banana.
Mash one large, ripe banana and measure to be sure it equals 1/2 cup, but no more. Add part of another banana if necessary. (In quick bread recipes like this one, it’s important not to add too much fruit, as it can make the bread too dense and heavy and can cause it to remain un-cooked in the middle.)
Add mashed banana and vanilla extract to the sugar mixture, then add buttermilk alternately with gfJules Muffin Mix and mix well, beating one minute after blended. Stir in nuts, if using.
Pour batter into the large loaf pan, filling 2/3 full; add remaining batter to mini loaf pan or muffin tins.
Bake large loaf for 60 minutes, or until a toothpick inserted into the center comes out clean; bake mini loaf pans 30 minutes; bake small muffins 15-20 minutes; bake extra large muffins 20-25 minutes.
The bread will be lightly browned and should spring back to the touch. If using convection, turn bread around half-way through the bake cycle to keep it rising evenly, and check for doneness 5 minutes before the times listed above, as breads tend to bake faster with convection.
Remove to cool on a wire rack for 15 minutes, then run a butter knife around the inside of the loaf pans and invert gently to remove the bread. Store in zip-top bags or tupperware containers on the counter, not in the refrigerator.