Preheat your oven to 220°C (430°F). Lightly grease your muffin tin or tartlet tray with oil. Unroll the sheet of puff pastry onto a lightly floured surface. If needed, gently roll it out to about 3mm thickness. Cut the pastry into circles about 10cm in diameter. Press each circle into the cups, forming little bowls. Refrigerate the tray while preparing custard.
In a medium saucepan, combine 250g sugar and 85ml water. Stir over medium heat until sugar dissolves. Bring to a gentle boil and cook about 3 minutes without stirring until it thickens slightly. Remove from heat and set aside.
In another saucepan, pour 350ml milk. Add cinnamon stick and lemon zest. Warm over medium heat until just about to boil, then remove from heat. Let steep for 10 minutes.
In a mixing bowl, whisk 6 egg yolks until smooth. Gradually add 50g flour and whisk until fully incorporated and lump-free.
Remove cinnamon stick and lemon zest from milk. Slowly pour warm milk into egg yolk mixture, whisking constantly. Pour mixture back into saucepan. Add sugar syrup and 5ml vanilla extract. Cook over low-medium heat, stirring constantly, until custard thickens in 5–7 minutes. Do not boil. Remove from heat immediately.
Pour custard evenly into prepared pastry cups, filling about 3/4 full. Bake at 220°C (430°F) for 15–18 minutes until pastry is golden and custard has dark spots.
Let tarts cool in tin for 5 minutes, then transfer to cooling rack. Serve slightly warm or room temperature. Sprinkle with cinnamon powder or powdered sugar before serving.
