Prepare the Maple Pecan Filling: Combine the maple syrup, brown sugar, heavy cream, and egg yolk in a small saucepan, stirring until the ingredients are well combined.
Cook the Filling: Add the cubed butter to the saucepan and bring the mixture to a very light boil, then simmer for 5 minutes, or until the mixture has lightly thickened.
Add the Pecans: Remove the saucepan from the heat and stir in the 1 cup of chopped pecans until evenly incorporated.
Cool the Filling: Allow the maple pecan mixture to cool completely on the kitchen counter, or refrigerate it for 30 minutes for faster cooling and additional thickening.
Preheat the Oven: Heat the oven to 180°C / 355°F with the rack positioned in the middle.
Prepare the Puff Pastry: If using pre-rolled puff pastry, proceed with the next step; if using puff pastry dough in thick square form, roll each piece separately on parchment paper to approximately 1/8 inch (2–3 mm) thick and about 10 x 15 inches (25 x 35 cm).
Cut the Pastry: Cut each sheet of puff pastry into 4 rectangles, approximately 5 x 7.5 inches (10 x 16 cm), creating 8 rectangles in total.
Fill the Pastry: Spoon approximately 2–3 tablespoons of the cooled, thickened maple pecan filling into the center of each pastry rectangle, leaving about ½ inch of space at the top and bottom and 2½ inches along the sides.
Cut the Plait Slits: Use a sharp knife or pizza cutter to make diagonal slits along both sides of the pastry next to the filling.
Shape the Plaits: Fold the pastry strips inward over the filling, overlapping them to create the plaited appearance, then tuck the top and bottom edges of the pastry inward.
Prepare for Baking: Arrange the first four assembled pastries on a parchment-lined baking sheet, then brush their tops with the beaten egg wash and sprinkle with the remaining chopped pecans.
Bake the First Batch: Bake the pastries at 180°C / 355°F for 18–20 minutes, or until the puff pastry is golden brown.
Bake the Remaining Pastries: Repeat the egg-washing and baking process with the second batch of four pastries.
Cool the Pastries: Transfer the baked plaits to a wire cooling rack and allow them to cool before serving.
Adjust a Runny Filling: If the maple pecan filling remains runny, add 1 tablespoon of cornstarch to the simmering mixture and cook for an additional 1 minute.
