Puff Pastry: In a large bowl or stand mixer, combine the 6 Tbsp butter and water. Heat it up if it doesn't mix well. Small floating clumps of butter is okay. Mix in the salt and flour.
Knead the dough on a lightly floured surface or use the dough hook until the dough unsticks from the surface and can be formed into a smooth ball. Cover in wrap and refrigerate for an hour (cool to touch).
Meanwhile, prep your butter. It can be helpful to start near room temperature. Cut long slices from the butter and piece together an approximate 8 inch x 8 inch square. Wrap in plastic or parchment and use a rolling pin to roll out the bumps to create a smooth square. Refrigerate for 30 min.
The dough and butter should be of similar pliability, not too stiff. Remove both from the fridge and place the dough on a slightly floured surface, rolling it out to a 12 inch x 12 inch square.
Place the butter block in a diamond (45 degrees off from the dough) and fold the corners of the dough to cover the butter block completely. You should end up with an 8x8 inch block of dough wrapping the butter.
Roll out the pastry dough to 16 inch x 24 inch. Fold it in thirds like a letter. Turn it 90 degrees. Roll it out to 8 inch x 24 inch. Fold into thirds again. Wrap and refrigerate for 30 min.
Repeat the last step twice more for a total of 6 folds, chilling between every second fold. Let it chill for an hour.
Pecan Filling: While the dough is chilling, make the filling. Combine the 2 Tbsp butter, brown sugar, maple syrup and 2 eggs in a pot. On low heat, whisk until the mixture thickens slightly. Be careful, once it starts, it happens very quick. Remove from heat and cool to room temperature before chilling in the fridge.
Meanwhile, prep the puff pastry. Roll it out to 8 inch x 12 inch. Divide it in half. Cover and refrigerate one half. For the other, roll out to 10 inch x 10 inch. Using a 9 inch circular pan, score the dough to mark the edges of the pithivier. Refrigerate and repeat with the second half. Do not cut the discs out.
Depending on your preference, you can start drawing lines with a knife to create a design now or after it's assembled. Many traditional patterns start by scoring from the center of the circle, radiating outwards.
Preheat the oven to 425°F. Line a baking sheet with parchment paper.
Assembly: Place the lower half of the pastry on a parchment lined baking sheet. Mix in the pecans into the filling and spread in the center of the pastry, leaving a 1-2 inch border from the edge of the marked circle. Gentle center and place the top pastry above everything. Using the 9 inch circular pan, cut out the pithivier. Score the top pastry if you haven't yet. Refrigerate as needed.
Make the egg wash by beating 1 egg with 1 Tbsp of water.
Use the egg wash and brush the pastry with it. Refrigerate for 10 min and repeat before placing the pastry in the oven. Bake for 20 minutes. Lower to 350°F and bake for another 30 minutes until golden brown and puffy.
Make the simple syrup. You can simply microwave it for 30 seconds at a time, until the sugar dissolves or melt it over the stove.
When the pithivier is baked, remove from the oven and immediately brush with the simple syrup. Cool to room temperature and serve. Enjoy!
Notes
Whenever you are not actively working with any part of the dough, cover and refrigerate it, especially if the kitchen is warm.