Mill flour ahead of time so that it has time to cool down.
In a large bowl combine flour, salt, and sugar.
Using a grater or a food processor, shred the cold butter.
Add the butter into the bowl with flour and combine using your hands or a pastry cutter.
If using a pastry cutter, cut the butter into the flour. If using your hands, work the butter into the flour until it resembles crumbs.
Pour the cold water into the flour and butter mixture, and mix with your hands until the dough comes together.
Dump the dough out onto a floured surface and use a rolling pin to roll into a rectangle. The dough should be about ¼ inch thick.
Cut off any uneven edges so that you have a smooth and even rectangular shape, about 12 x 18 inches.
Cut the dough in 3-4 inch strips. You should get about 4-5. Then, cut those strips in half to make your poptart shapes.
Repeat this process until you use all the dough.
Fill half of your rectangles with the strawberry jam - about 2-3 teaspoons. Make sure not to overfill so the jam doesn’t spill out while baking.
Place the other half of the rectangles on top of the ones with jelly and crimp the edges with a fork.
Line a baking sheet with parchment paper and preheat the oven to 425 degrees F.
Place prepared pop tarts on a baking sheet and refrigerate for 20 minutes.
Bake in the oven for 10 minutes. After 10 minutes, rotate the baking sheet and cook another 12 minutes.
While the pop tarts cool, prepare icing by combining all ingredients together. The icing should not be very runny. If it is, add some more powdered sugar.
Spread icing over cooled pop tarts.