Add the flour, sugar and salt to a food processor and pulse a few times to evenly combine. Add the butter and shortening and pulse about 8 times, just enough to cut butter into smaller pieces. With the processor running, pour ice cold water down the feed tube and pulse until the dough begins to clump together.
Turn the dough onto a floured surface and gather into a ball. Divide into 2 even portions and flatten or roll each portion into an approximate 6-inch disc and wrap tightly with plastic wrap. Chill at least one hour in the refrigerator.
ASSEMBLY
Line two baking sheets with parchment paper and set aside. Remove 1 disc of dough from the refrigerator and let rest 10 minutes to soften (keep the other disc in the refrigerator).
On a lightly floured cutting board, roll the dough into a little larger than a 9x12-inch rectangle in order to create nine 3x4-inch rectangles. Using a ruler and a pizza cutter, cut three 3-inch rows across the 9-inch side. Across the 12-inch side, cut four 4-inch rows. Transfer the 9 rectangles to one of the baking sheets (space them evenly) and place in the refrigerator. Repeat with remaining dough to create 9 more rectangles then transfer them to the other baking sheet.
To these rectangles (the second set), brush egg wash over all of the edges. Add 1 tablespoon Nutella down the center of each rectangle (it will spread out when baked), leaving at least a ½ inch dough perimeter. Whisk 3 tablespoons brown sugar and 1 teaspoon cinnamon together in a small bowl then evenly sprinkle over Nutella centers.
Remove the other baking sheet from the refrigerator and brush the entire surface of all of the rectangles with egg wash and place them WASH SIDE DOWN over each Nutella rectangle. Press the edges firmly together all the way around to seal in the Nutella. Use the tines of a fork to double-seal the perimeter of each of the pastries. Prick the top of each pastry several times with a fork (so steam can escape while baking), or use a toothpick to prick about 8 times. Freeze pastries for 30 minutes or refrigerate up to 1 hour. Refrigerate remaining egg wash.
When ready to bake, brush the tops of each pastry with remaining egg wash. Bake at 350 degrees F. for 25 to 30 minutes or until lightly golden, rotating once while baking. Let the pop tarts cool for 5 minutes then transfer to a wire rack to cool completely before glazing.
GLAZE
Whisk together all of the glaze ingredients until smooth, adding a teaspoon more milk at a time if needed to reach spreadable consistency.
When pop tarts are cool, spread glaze over the top. Glaze will begin to harden after 30 minutes, but I can never wait that long. Store in an airtight container.
Notes
After you chop your butter into ½ inch pieces or smaller, return to refrigerator to chill.