Place flour, powdered sugar, butter, vanilla, and salt, in a food processor. Pulse until the mixture resembles a coarse meal. Add the egg whites and process just until a uniform dough is obtained. If the dough is not coming together, add a little water (1-2 teaspoons).
Shape the dough into a disk about 1 inch thick before chilling. Wrap the dough in plastic wrap and transfer it to the refrigerator for 1 to 2 hours (up to overnight).
Roll the dough on a floured surface to a ¼ -inch (½ cm) thickness and cut circles using a cookie cutter. Place a flat teaspoon of the chocolate spread and a few candies. Pinch the edges together in three places so that they meet in the middle to form a triangular cookie shape. Pinch the three corners of the triangle together firmly to seal the edges. Make sure the filling is well-enclosed.
Arrange the Hamantaschen on a baking sheet lined with parchment paper. Place the baking sheet in the freezer for 30 minutes to an hour before baking.
Preheat the oven to 350°F/180°C.
Use a pastry brush to lightly coat the sides (edges) of each assembled Hamantaschen with the egg white. Prepare a plate or shallow dish with the rainbow sprinkles. Gently dip the edges of each Hamantaschen into the sprinkles. The egg white acts as a glue, allowing the sprinkles to stick to the surface of the dough.
Bake for 15-20 minutes until the Hamantaschen is slightly golden. Let them cool and enjoy.