In the bowl of a stand mixer fitted with the paddle attachment combine flour, salt and sugar. Add the butter and mix on medium high until it resembles a coarse meal.
Add 5 tbsp of ice water then mix again to bring together, it should form larger clumps that hold together when pressed. If it is still too crumbly add another tbsp of ice water and mix again. Gather into a ball then divide in half, shape each half into a disk and cover with plastic wrap. Chill in the fridge for an hour.
Line a baking tray with parchemnt and set aside. Working with one chilled disk at a time roll out the dough onto a lightly floured surface into a large rectangle. Use a sharp knife to trim the edges so they are even and you are left with a rectangle that is 12x10 inches. Cut the rectangle in half lengthwise so you have two long strips. Cut each strip into four pieces, you should have 8 rectangles total that are 5x4 inches in size. Place onto prepared baking sheet.
Repeat this process with the second disk of dough so you have 8 rectangles to top each tart. Add 11/2 tbsp. of jam to the center of each tart on the baking tray, use the back of the spoon to spread jam an even layer leaving a border around the outside.
Top each tart with the remaining 8 rectangles and use your fingers to seal the edges of the tart and the two pastry layers together. Then use the tines of a fork to press around all the edges ensuring the jam is well sealed inside. Use a toothpick to pok a few holes into the tops of each tart. Place the tray of tarts into the freezer to chill for at least two hours, up to overnight.
Preheat the oven to 375 degrees then place the frozen tarts into the oven to bake for 25-30 minutes, turning the tray halfway through the baking time. Once time is up, and tarts are nice golden brown, remove from the oven and allow to cool before frosting.
To prepare the frosting add the icing sugar, vanilla, and milk into a bowl then stir to fully combine the ingredients so you have a nice smooth but thick icing consistency. If the icing is too thick you can add more milk, only a tablespoon at a time, to achieve deisred consistency.
Once the pop tarts are cooled spread an even layer of icing over the top of each tart, leaving a border around the outside. Then add sprinkles to the tops of each tart if desired. Allow the icing to set before serving or storing.