Put a bowl in the freezer (you’ll need this to cool down the cream).

In a saucepan over low heat, combine the milk, vanilla extract and lemon zest. Heat until the milk is scalding hot.

While the milk is heating, whisk together the egg yolks, sugar and corn starch until the mixture forms a smooth paste.

Temper the eggs by pouring a ladleful of the scalding milk into the egg mixture and whisking continuously. Add a couple more ladles of milk to the eggs until the mixture becomes pourable.

Add the tempered egg mixture back into the saucepan. Stir continuously with a spatula until the cream thickens. Make sure to allow the cream to reach a temperature close to boiling (you should see a couple of bubbles forming around the edges) to ensure proper thickening.

When the cream reaches an almost pudding-like consistency, transfer it to the bowl that’s been cooling in the freezer. Stir vigorously with a spatula until the cream becomes thick, shiny and uniform. Place a piece of plastic wrap against the surface of the cream (to prevent a skin from forming) and set aside to cool completely. Once cooled, chill in the refrigerator for at least 3 hours.

Combine the flour with a pinch of salt in a small bowl. Rub in the butter using your hands, a fork, or a pastry cutter until it has a sandy consistency. Mix in the powdered sugar, followed by the egg and about 1 tablespoon of cold water. Continue mixing until the dough starts to come together. Do not overmix.

Shape into a ball, cover tightly with plastic wrap and chill for at least 30 minutes.

Grease a standard muffin tin (or use mini tart tins if you have them) and dust with some flour.

Cut a piece of shortcrust pastry and roll it into a ball about the size of a walnut. Using a shot glass or cork, press the dough into the muffin pan and up the sides. Prick the bottom and sides with a fork, line with a piece of baking parchment, and fill with baking weights (e.g. dried beans or rice). Repeat for all 12 pastry shells, and place them in the freezer for about 15 minutes.