You should end up with 24 pieces (yielding 12 Pop-Tarts), but if you don't, that's fine! Just aim for an even number and try to keep the dough relatively thin.
2 c (283 g) flour
1 tsp salt
1 c (226g) butter, cubed and frozen for at least 15 minutes
3/4 c (177 ml) corn syrup
Combine the flour and salt in a large bowl. Cut in the butter until the mixture consists of pea-sized lumps. Add the corn syrup and mix just until it comes together. Dust your hands with flour, scoop out the dough, and knead lightly until smooth.
Flatten the dough into a square, wrap in plastic wrap, and chill for at least 30 minutes, up to three days.
Lay a piece of parchment or wax paper on your counter and dust with powdered sugar. Roll the chilled dough to 1/4” thick, maintaining a square shape by refraining from rolling diagonally. Lift and move the dough periodically to ensure that it does not stick, and add more powdered sugar underneath if necessary.
When the dough is rolled to the appropriate thickness, use a ruler to cut it into as many 3 1/8” wide strips as you can. Then, cut each strip every 4 inches. Remove scraps and transfer parchment/wax paper to a cookie sheet. Place in the refrigerator while you re-roll scraps in the same fashion, refrigerating until needed.
After preparing the filling (recipe below), Pipe a thin layer of fruit filling on half (12) of the rectangles, leaving a 1/2" border on each side. Keep in mind that Pop-Tarts are not FILLED with fruit as you work, and try to keep the layer relatively smooth.
Brush the border lightly with water and place another layer of pastry over the fruit filling. Seal the edges by lightly pressing them with the rounded edge of a bench scraper. Dock each Pop-Tart about 8 times (2 rows of 4).
Preheat the oven to 350F and refrigerate the finished tarts while the oven comes to temp.
When the oven is ready, bake the Pop-Tarts for about 20 minutes, until lightly browned but not golden brown. If the Pop-Tarts spread while baking, tap/trim them back into shape with a bench scraper or knife while they are still warm.
Cool completely before frosting.
Cranberry and Orange Filling
Stella provided some wonderful ideas for filling over on her blog, but I was low on dried fruit and too lazy to go shopping, so I worked with what we had to create a simple, tasty filling.
1 2/3 c (266 g) dried cranberries
1 orange, zest
2 Tbls corn syrup
Place the dried fruit and corn syrup in a food processor. Blitz until a thick paste is formed, letting the machine run for a minute or so to ensure no large pieces remain.
Transfer the fruit filling to a pastry bag fitted with a large petal tip.
Be sure to keep this covered with a damp towel when you are not using it or it will crust over.
2 1/8 c (340 g) powdered sugar
2 egg whites
1/2 tsp salt
2 tsp vanilla
Sprinkles, homemade or store-bought
Place all of the ingredients except the sprinkles in a large bowl and beat until stiff and glossy, about 10 minutes. The resulting icing should be the perfect consistency for piping, but if not, beat in more powdered sugar until it is.
Using a piping bag fitted with a small, plain tip, pipe a border around the edge leaving a 1/4" margin. Allow to dry for about an hour.
Thin remaining icing by adding water to it in 1 tsp increments until a trail of icing from a spatula disappears in 3 seconds. Flood the border you piped earlier with a small amount of icing, using a small offset spatula or toothpick to reach to corners. Allow to dry 3 minutes before adding sprinkles, then overnight before eating.