Cut the passion fruits in half and scoop out flesh with a spoon. Put flesh of 1 passion fruit in a small bowl and reserve. Place remaining pulp in another bowl.
Separate the eggs. Beat egg whites in a tall vessel with a hand mixer until stiff, then gradually add 2/3 of the sugar while beating.
Gently fold the egg whites into the batter with a whisk and add sweetener to taste.
Place baking cups in the muffin tin, pour in the batter and gently smooth. Bake in preheated oven at 180°C (fan 160°C, gas: mark 2-3) (approximately 350 ° F) until golden brown, 20-25 minutes. Let cool completely in the tin on a wire rack.
Rinse berries, drain and cut into small pieces if necessary. Mix with reserved passion fruit pulp. Spoon over the muffins and serve.