1. Make the Pastry:

• In a large bowl, mix the flour and salt. Add the cold butter and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.
• Gradually add cold water, mixing until a dough forms. Do not overmix.
• Wrap the dough in plastic wrap and refrigerate for at least 1 hour.

2. Prepare the Custard Filling:

• In a saucepan, combine the cream, milk, sugar, cornstarch, cinnamon stick, and lemon zest (if using). Heat over medium heat until just boiling, whisking constantly to dissolve the sugar and cornstarch.
• In a separate bowl, whisk the egg yolks until smooth. Gradually add the hot milk mixture to the yolks, whisking continuously to prevent curdling.
• Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens (about 2-3 minutes). Remove from heat and stir in vanilla extract.
• Let the custard cool for about 15 minutes, then cover with plastic wrap to prevent a skin from forming.

3. Roll Out the Pastry:

• Preheat the oven to 475°F (245°C).
• On a lightly floured surface, roll out the chilled dough into a rectangle about 1/8 inch thick.
• Fold the dough into thirds (like a letter), then roll it out again to the same thickness. Repeat this process two more times. This creates layers for a flakier pastry.
• Cut out circles from the rolled dough to fit into a muffin tin (about 3 inches in diameter).

4. Assemble the Tarts:

• Grease a muffin tin with butter. Press the pastry circles into each muffin cup, making sure the sides are higher than the edges.
• Fill each pastry shell with the custard filling, leaving a little space at the top.

5. Bake:

• Bake in the preheated oven for about 15-20 minutes, or until the edges are golden brown and the tops are slightly caramelized. Keep an eye on them to avoid burning.

6. Cool and Serve:

• Once baked, remove from the oven and let the tarts cool in the tin for a few minutes. Then transfer them to a wire rack to cool completely.
• Serve warm or at room temperature, optionally sprinkled with powdered sugar or cinnamon.

Tips:

• For an extra crispy pastry, you can freeze the pastry shells for 15-20 minutes before filling and baking.
• These tarts are best enjoyed the same day, but they can be stored in an airtight container for up to 2 days.