Whisk together the flour, salt and baking powder in a large bowl.

Add the cubed butter to the bowl and use your fingers to work the butter into the flour until the mixture is the consistency of wet sand.

Stir in the sour cream (the dough will be very wet), and then turn the dough out onto a well-floured work surface.

Knead the dough a few times until it comes together, adding more flour, 1 teaspoon at a time, if the dough is too sticky to handle. Roll the dough into an 8x10-inch rectangle and dust both sides with flour before folding it into thirds as if you were folding a letter.

Rotate the dough 90º and roll it out again into an 8x10-inch rectangle. Fold the dough again into thirds then wrap it securely in plastic wrap and refrigerate it for at least 30 minutes while you make the filling.

In a small bowl, combine the diced strawberries with the flour, tossing to combine. Set aside.

Preheat the oven to 425ºF.

Remove the dough from the refrigerator and unfold it onto a well-floured surface, rolling it out to a 14x14-inch square. (See Kelly's Notes.)

Using a 3-inch circular cookie cutter or cup, cut out as many circles as possible. Re-roll the scraps and continue cutting out circles until you run out of dough, ensuring you end up with an even number of circles. (You should have approximately 20 3-inch circles.)

Place six of the dough circles on a Silpat or parchment paper-lined cookie sheet. Spread a portion of the Nutella in the centers of half of the dough circles leaving a 1/4-inch border around the edges. Spoon a small portion of the strawberries atop the Nutella. Place a second dough circle atop each filled circle then use a fork to crimp the edges together, sealing each pie.

Whisk together the egg with the water to make an egg wash. Brush each pie with the egg wash, and using a sharp knife, cut two or three vents on the top of each pie. Sprinkle the pies with crystal sanding sugar (optional).

Bake the pies for about 15 minutes, or until they're golden brown.

Remove the pies from the oven and allow them to rest for 10 minutes before serving. Repeat the filling and baking process with the remaining dough circles.

The diced strawberries should be no larger than the size of corn kernels to ensure they become tender during the quick 15-minute bake time.

If the dough is too firm to work with after refrigerating, allow it to sit at room temperature for 10 minutes before cutting out the circular shapes.

If the sour cream in the dough makes it too sticky at any point in the rolling out process, just simply sprinkle a pinch of flour atop the wet areas and continue rolling.