In a large mixing bowl with the paddle attachment, add the flour, sugar, and salt. Mix on low speed and slowly add the butter cubes. Increase speed to medium and continue until butter breaks down to pea size. Whisk together the water and egg yolk in a small bowl and add all at once to the mixer. Mix on low speed just until dough comes together. Form dough into two discs and wrap each in plastic. Let chill in refrigerator for 1 hour.
Line a sheet tray with parchment paper. Set aside another piece of parchment of the same size.
On a lightly floured surface, roll out one disc of dough into a 13 × 11-inch rectangle. Trim down to 12 × 10 inches with a sharp knife and then cut out eight 5 × 3-inch rectangles. Place on prepared sheet tray and cover with the extra parchment sheet. Repeat this step with the remaining disc of dough, place cutouts on the top parchment sheet, and then place sheet tray in refrigerator for about 20 minutes to chill the dough cutouts.
Slide off the top layer of parchment and its cutouts. These will be the tops of the pop tarts.
Poke some holes into the bottom cutouts with a fork. Whisk the egg white and, using a pastry brush, moisten their edges. Spread a tablespoon of Nutella down the center. Lay a top cutout over the filling and rub your fingertips around the edges to seal. Use the tines of the fork to double seal the edges and poke some holes on top. Place sheet tray in freezer for 1 hour.
Preheat oven to 375°F. Bake for 15 minutes. Cool completely. Garnish with sifted powdered sugar before serving.