Preheat the oven to 400°F. Line a rimmed sheet pan with parchment paper.

Make the almond filling. Add the almond paste, softened butter, sugar, egg yolk, vanilla, almond extract if using, and salt to a bowl or food processor. Mix until smooth, thick, and creamy. If the filling feels too loose to pipe or spoon, mix in 1 to 2 tablespoons flour.

Make the egg wash. Whisk together the egg, milk or water, and salt in a small bowl.

Prepare the puff pastry. Lightly flour the work surface. Unfold the pastry sheets and gently roll just enough to smooth the seams. Cut each sheet into 4 rectangles for 8 total rectangles.

Add the almond filling. Pipe or spoon a thick line of almond filling down the center of each rectangle, leaving room on both sides for folding.

Make the pastries two ways. For classic almond pastries, leave the almond filling plain. For almond chocolate chunk pastries, add chocolate chunks or chocolate chips over the almond filling before folding. You can make half classic and half chocolate, or make the whole batch one way.

Fold the first side of pastry over the filling. Brush the outside edge of that folded flap with egg wash.

Fold the second side over the first so the pastry overlaps. Brush a little egg wash underneath the second flap as well. Press gently along the overlap so the pastry sticks. This helps the pastries stay closed as the puff pastry expands in the oven.

Brush the tops with egg wash and sprinkle generously with sliced almonds.

Chill the tray for 10 to 15 minutes if the pastry feels soft or warm.

Bake for 18 to 22 minutes, until the pastries are puffed, golden brown, crisp on the edges, and the almonds are toasted.

Cool on the pan for 10 minutes before serving. Dust with powdered sugar if desired.