Add the custard powder and caster sugar to a medium saucepan and whisk together.

Pour in ¼ cup (60 ml) of the milk and whisk until completely smooth with no visible lumps.

Add the remaining 1 cup of milk and the finely grated lemon zest. Whisk until combined.

Place the saucepan over medium-high heat. Cook for approximately 8–10 minutes, whisking constantly, until the custard begins to simmer and becomes thick and creamy.

Remove the saucepan from the heat and whisk in the fresh lemon juice.

Transfer the lemon custard to a heatproof bowl. Press plastic wrap directly against the surface to help prevent a skin from forming. Leave it to cool for approximately 30 minutes.

Preheat the oven to 350°F (180°C), or 320°F (160°C) for a fan-assisted oven.

Place one cookie over each cavity of round-bottomed shallow muffin or patty pans. Depending on cookie size and your pans, the recipe should make approximately 20–21 mini tart shells.

Bake the cookies for about 3 minutes, just until they become warm and soft enough to shape.

Working quickly while the cookies are still pliable, carefully press each softened cookie down into its muffin cavity using the back of a teaspoon to create a small tart cup.

Allow the cookie cups to cool and firm up.

Whisk the cooled custard until smooth, then evenly divide it among the cookie shells.

Refrigerate the Lemon Custard Tarts for approximately 3 hours, or until the filling is chilled and set.

Just before serving, top each tart with a small dollop of whipped cream and a quartered lemon slice.