In a small bowl, combine the water and cornstarch. 

Add the strawberry jam and cornstarch mixture to a saucepan and whisk to combine. 

Bring to a boil and simmer for 2 minutes, stirring constantly. 

Remove from the heat and set aside. 

In a small bowl whisk the egg and sour cream. 

Add the flour, salt and sugar to a food processor and pulse a few times to combine. 

Add the cubed butter to the food processor and pulse a few times until coarse crumbs form.

Add the sour cream mixture and pulse a few times until the dough just forms into large clumps. 

Turn out the dough onto a floured surface and gently knead until it comes together.

Divide the dough into 2 balls. 

Flatten each ball into disk. 

Wrap each disc with plastic wrap and refrigerate for at least 2 hours. 

Remove the dough from the refrigerator and let stand for 15 minutes at room temperature. 

On a lightly-floured surface, roll out one disk to about ⅛-inch thick, in a 9½ by 12½ rectangle.

Cut into nine 3×4 inch rectangles. 

Place the rectangles onto parchment-lined baking sheet.

Place 1 tablespoon of cooled jam mixture in center of each bottom rectangle leaving a ½-inch of space around the edges.

Place the baking sheet in the refrigerator to chill while rolling second disk of dough.

On a lightly-floured surface, roll out the dough to about ⅛-inch thick, in a 9½ by 12½ rectangle.

Cut into nine 3×4 inch rectangles. 

Remove the baking sheet fro the refrigerator. 

Place 1 top rectangle onto each bottom rectangle, making sure all edges are lined up. 

Use a fork to seal the edges.

Prick the tops of the rectangles with a fork. 

Refrigerate the pop tarts for 30 minutes. 

Preheat oven to 350ºF.

Bake in preheated oven for 25 to 30 minutes, or until lightly browned.

Remove from the oven and allow to cool completely. 

In a small bowl, combine the powder sugar, milk and vanilla extract. 

Spread a spoonful of icing over each pop tart. 

Top with sprinkles and allow to sit for 30 minutes so the icing can set.