Preheat oven to 350ºF. In a small pot over medium-high, combine jam and cornstarch mixture; bring to a boil. Reduce heat to medium-low and simmer for 2 minutes; remove from heat and let it cool.
Roll 1 dough to a rectangle and cut pieces of 5 ½ x 3 ½-inches, making 5 tarts (or cut smaller if desired). Transfer to a baking sheet lined with a silicone mat; brush egg wash and scoop jam mixture leaving a border around the edges.
Roll the 2nd dough to a rectangle and cut pieces slightly bigger than the first ones. Place them over the jam mixture and the bottom dough, pressing down around the edges. Roll up the bottom and top dough together preventing the filling from escaping. Use the tines of a fork to crimp the edges of the tarts. Brush the top with the remaining egg wash and poke some holes of top to let the steam escape.
Transfer to preheated oven and bake until crust turns light golden brown, about 25 to 30 minutes. Remove from the heat and cool completely.
To make icing, combine all the ingredients together in a small bowl. When tarts are at room temperature, spoon icing on top and spread it with the back of the spoon. For the finishing touch, add colorful sprinkles.