Add the Keto Bread and Pastry Flour to a bowl along with the sweetener. Whisk together.
Add the cold butter pieces.
Stir to coat the butter with flour.
Use a pastry cutter or fork to break the butter into smaller pieces.
Add the cold water and stir until a shaggy dough forms.
Form the dough into a rough ball using your hands (it's okay if it's a little dry, just push everything together and gather up as much flour as you can from the bowl to form the ball).
Wrap the dough in plastic wrap and refrigerate for 30min - 1 hour.
Un-wrap the dough and place it onto a floured piece of parchment paper (use additional keto flour or vital wheat gluten to flour the surface).
Flour the top of the dough and place another piece of parchment paper on top.
Use a rolling pin to roll the dough out into a rough rectangle. The butter will be quite chunky and messy right now, this is okay.
Fold one end of the rectangle inward then fold the other inward as if you were folding a brochure (see the video for more help).
Turn the dough 90*. Flour the surface as well as the top of the dough and again, roll the dough out between the two pieces of parchment paper into a rectangle. Repeat step 10 once more.
Wrap the dough in plastic wrap and place it in the fridge to chill for at least 10minutes (30 is best if you have the time).
Un-wrap the dough and repeat steps 9 - 12 again (the dough should get smoother as you go and the butter will become more streaky).
After the second refrigeration, repeat steps 9-12 one more time for a total of 6 stages of rolling and folding.
After 10-30min in the fridge, un-fold your dough. Roll it out into one large rectangle this time (roll both width-wise and length-wise). It should be relatively thin but still thick enough that it won't easily tear. (Don't worry if your rectangle is not perfect).
Slice the dough into 4 smaller rectangles and slice each one in half to form squares.
Place a tablespoon of hazelnut spread in the center of one of each of the halves.
Place the other piece of dough on top of the piece with hazelnut spread and gently press the edges together with your fingers. Then use a fork to go around and seal the edges of the two pieces together.
Poke 2 fork holes in the top of each pastry then brush with the egg wash (1 egg yolk whisked with 1 tbsp water).
Place the pastries on a parchment-lined baking sheet and bake for 20-22min or until puffed and golden brown. (TIP: Wrap the baking sheet with plastic wrap and refrigerate your pastries for 20-30min prior to baking to prevent butter from leaking out as they bake-optional).
Let the pastries cool for a few minutes before serving. Top with a simple glaze if you wish (optional). Just whisk together the powdered monkfruit sweetener and milk until smooth and spoon over each pastry. The glaze will set as the pastries cool. Enjoy your Keto Nutella Puff Pastries!