Preheat the oven to 425 degrees F.
In a small saucepan, bring the jam and cornstarch mixture to a boil over medium heat. Reduce the heat to a simmer and cook stirring for 3 to 4 minutes. Set aside and let cool to room temperature.
Lay 1 sheet of pastry on a board and cut it into 4 equal strips. Put a heaping teaspoon of cooled jam in the middle of the upper part of the dough and fold over the bottom part. Seal the edges with a bit of water and crimp with a fork. Arrange the pastries on a parchment lined baking sheet. Repeat with the other pastry sheet. Bake until golden about 8 to 10 minutes. Remove the pastries from oven and allow them to cool completely.
In a small bowl, add the powdered sugar and whisk in, 1/2 tablespoon of lemon juice at a time, to make a thick icing consistency. Drizzle the icing over the pastries and sprinkle with colored sugar, if desired. Allow the icing to set for about 30 minutes. Enjoy at room temperature.
Recipe courtesy of Claire Robinson, 2010