Preheat oven to 300°F. In a large bowl, whisk together the buttermilk, eggs, and 2 tbsp butter. Add the pancake mix and stir until just combined (some lumps are OK).
Lightly brush a 12-inch nonstick skillet with some of the oil and heat on medium. Add the batter, ¼ cup at a time, to form pancakes and cook 1–2 min., until edges begin to set and bubbles start to form. Turn pancakes over and cook another 1 min., until bottoms are golden brown. Add cooked pancakes to a baking sheet and keep warm in oven. Repeat with remaining oil and batter.
When pancakes are done, wipe out skillet with a paper towel. To same skillet, add the brown sugar and remaining 5 tbsp butter. Cook on medium 2–3 min., until bubbly, stirring occasionally.
Add 2 tbsp water and the bananas. Cook 3–4 min., until bananas are soft, stirring constantly. Stir in the walnuts and remove from heat. To serve, spoon bananas over pancakes. Top with the whipped cream, if desired.