For the dough: In a large bowl, mix the butter, flour, salt, lemon juice and 2-3 tablespoons of water. Gently knead the dough in a cool place between two plates, making sure one of the plates is covered with damp cloth. The dough should be soft but not sticky.
Roll out the dough and cut into 6 pieces.
Peel the plantain, cut lengthwise and spread with strawberry jam.
Roll the plantains in the dough and brush with egg white on the ends so that dough sticks.
Brush again with with egg white and sprinkle with vanilla sugar. Preheat oven to 230°C (approximately 450°F) and bake for about 15 minutes.
Serve hot or cold.