Preheat a large frying pan on medium-high. Preheat the oven to 200° or warm.
In a medium bowl, combine the flour, sugar, baking powder, and salt. In a small bowl, whisk the melted butter, eggs, and milk to combine. Add the wet ingredients to the dry and whisk until combined.
Place a dab of butter into the pan, and swirl it around to coat. Pour or scoop a quarter cup of batter onto the pan. Cook three pancakes at a time. Once you see tiny bubbles start to form, flip the pancakes and continue cooking for 1 minute or until brown. Place the pancakes in the oven to keep warm.
Once all of the pancakes are cooked, wipe the frying pan clean. The temperature of the burner should be still set to medium high from the pancakes.
Place 2 tablespoons of butter into the pan and heat until it begins to bubble. Stir in the brown sugar. Add the vanilla and the milk slowly. The sugar will pop and sizzle. Stir until you have a nice thin caramel. Add the sliced bananas and walnuts and coat with the caramel sauce. Continue cooking for a few minutes until the sauce has thickened slightly and the bananas are soft.
Top the pancakes with the bananas foster, and serve warm.