Preheat oven to 400° and line a baking sheet(s) with parchment paper. Set aside.
In a medium bowl, whisk flour, sugar and salt together.
Add egg whites and vanilla, stir until nice and smooth.
Next, add butter and whisk until combined.
Using a 1 1/2 tsp cookie scoop, drop onto prepared baking sheet 4 inches apart.
Carefully place in the oven and bake for approximately 5-7 minutes or until edges start to brown.
Remove from oven, loosen from baking sheet and using a wooden spoon, wrap the cookie around the end of the spoon. Press to seal and hold for 15-20 seconds.
Once set, remove from end of wooden spoon and place of wire rack to cool. Continue with remaining cookies. Let cool completely before filling. *Return to oven for 1 minute if cookies become too cool and unable to shape.
Beat heavy cream on high speed until high peaks form.
Add powdered sugar and beat until soft peaks form and mixture is light and fluffy.
Fold mixture into mashed bananas.
Stir lemon juice and rum into mousse mixture. Cover with plastic wrap and chill.
Fill cookies once cookies have cooled completely then enjoy!