Cook's Notes: You don't need to go out and purchase older egg whites, you can simply used the eggs that you've had in the refrigerator for the last 4 or 5 days. After you separate the eggs, make sure you allow the egg whites to come to room temperature before whipping. Room temperature egg whites whip better and faster if at room temperature.
For the meringue cookies: Preheat the oven to 300 degrees F. Prep 2 half sheet pans with parchment or silicone baking mats.
Sift together your almond meal and powdered sugar and set aside.
In the bowl of a standing mixer, or with an electric hand mixer, beat the egg whites on low speed until foamy. Begin sprinkling in the meringue powder as you beat. Then increase the speed to medium and beat until soft peaks form. If adding food coloring at the point, you can divide the meringue, add to a separate bowl and dye as desired (see Cook's Note, below). Beat to combine. When you get to soft peaks (the peaks will curl when the whisk is lifted from the mixture), carefully proceed; you want firm glossy peaks in your meringue, you do not want to beat all the way to stiff peaks (peaks stand up straight when a whisk is lifted from the mixture).
Add a third of the almond mixture, folding it in gently. Once combined, add the remaining almond flour in 2 batches. Be very, very gentle! Make sure to fully incorporate all your ingredients, but do not over-fold.
Spoon the mixture into a pastry bag fitted with a plain 1/2-inch tip. (You can also use a ziptop bag with a corner cut off.) It's way easier to fill your bag if you stand it up in a tall glass and fold the top down before spooning in the batter.
Pipe 1-inch mounds of batter onto the prepared sheet pans. Using a wet fingertip, tap the mounds to a smooth tip. Alternately, pick up the pan with both hands, and holding it level, tap it firmly onto the counter several times. Allow any residual water to dry out. Sprinkle each cookie with fleur de sel. Allow the cookies to rest 30 to 60 minutes, depending on the humidity.
Bake the cookies for about 15 minutes, 1 sheet at a time. Let the cookies cool.
For the filling: In a heavy bottomed saucepan set over medium-high heat, combine the sugar and 2 tablespoons water. Slowly let the sugar melt and turn into caramel (do not stir). Carefully add the butter and heavy cream and stand back as it will release a lot of steam. Stir until everything is fully incorporated. Add the fleur de sel.
In a small bowl, mash the ripe banana with a fork. Once the caramel has cooled, add it to the mashed banana. Spread the mixture on the cookies, making sandwiches. Place in the refrigerator and allow to mature for at least 24 hours.