Line two large baking sheets with parchment paper. Position a rack in the middle of the oven. Preheat the fan-assisted oven to 200 °C / 390 °F or the conventional oven to 210 °C / 410 °F.
sprinkle the puff pastry with sugar
Stir to combine the sugar with freshly grated orange zest in a bowl. Take your puff pastry out of the fridge. Unroll or if you're not using a prerolled puff pastry, roll it to a 2-3mm (1/8 -inch) thickness before using. Always keep your puff pastry cold. Brush with an egg yolk wash and sprinkle with half of the sugar mixture. Using a rolling pin, roll the pastry, pressing the sugar into the pastry to distribute it evenly. Turn the pastry around and repeat the process with the rest of the sugar mixture.
roll the pastry and cut into cookies
Start rolling the pastry with your fingers. Tightly roll the bottom of the pastry toward the middle, stopping at the center. Then roll the top of the pastry to meet in the center. The thickness of both rolls should be the same size. Cut in half, then cut into 34 1/4-inch (0.5 cm) cookies using a serrated knife or another sharp knife.
Place the cookies on the prepared baking sheets, leaving enough space to expand, and using your finger, press each cooking down slightly. Bake each sheet separately. Bake for 12 - 14 minutes in the fan-assisted oven to 200 °C / 390 °F or in the conventional oven to 210 °C / 410 °F. Turn the baking sheet once while baking. Remove from the oven and allow to cool.
coat in chocolate
Melt the dark chocolate in a microwave or a bowl over a pot filled with boiling water. Dip half of the Orange Palmier Cookies in melted chocolate and set aside for the chocolate to set.