Sprinkle the gelatin over the warm water and let it "bloom" for a few minutes, while you work on the rest of the recipe.
Place the chopped walnuts and dates on a cutting board and chop them together until they become well mixed in and have a texture that resembles a crumble. Reserve.
In a large mixing bowl, whisk the egg whites with a hand mixer on high speed until stiff peaks form; set aside.
In a separate mixing bowl, place 1-1/2 ripe banana, the plain Greek yogurt and the whey protein powder. Mix well with the electric mixer on medium speed this time, until completely combined and slightly frothy.
Place the gelatin in the microwave for about 15 - 20 seconds, until completely melted but not hot.
Add the gelatin to the banana mixture and stir until completely incorporated. Then, fold in the egg whites with a rubber spatula and stir delicately until completely incorporated.
Fill two large 16oz glass coffee mugs about halfway up, then sprinkle about one quarter of the date/walnut crumble on top of each. Save the rest to garnish later once the mousse has set. Next, slice the remaining half banana and arrange a few slices around the sides of each mug.
Top with the remaining mousse mixture and send to the fridge until completely set, about 4 hours (or better yet, overnight)
Garnish with a little bit of Greek yogurt, top with the remaining date/walnut crumble and a few more slices of bananas.