In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate medium bowl, combine buttermilk, egg, 3 tablespoons melted butter, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined; do not overmix.
Heat a lightly greased griddle or non-stick skillet over medium heat and pour 1/4 cup batter for each pancake, cooking until bubbles form on the surface, then flipping to cook until golden brown.
In a separate large skillet over medium heat, melt 4 tablespoons butter and stir in the brown sugar and cinnamon until bubbly.
Add the sliced bananas to the skillet and cook for 2 minutes, stirring gently to coat.
Carefully stir in the dark rum and heavy cream, simmering for 1-2 minutes until the sauce thickens slightly.
Stack the pancakes on plates and spoon the warm banana Foster sauce generously over the top.