Preheat the oven to 180°C (350°F). Thinly slice 2 lemons, including the peel, and remove any seeds.

Add 100 g caster sugar and 100 ml water to a saucepan. Bring the mixture to a gentle simmer over medium-low heat.

Add the sliced lemons to the syrup. Simmer gently for 25 to 30 minutes until the slices are translucent and candied.

Arrange the candied lemon slices in an even layer across the base of the 8-inch round cake pan. Spoon a little syrup from the saucepan over the slices.

Whisk 2 eggs, 150 g caster sugar, and the zest from half a lemon in a mixing bowl. Continue whisking until the mixture is pale, thick, and voluminous.

Slowly stream in 70 ml vegetable oil while whisking continuously. Whisk in 1 teaspoon vanilla extract until incorporated.

Sift 120 g all-purpose flour, 1/2 teaspoon baking powder, a pinch of baking soda, and a pinch of salt into the egg mixture. Fold gently with a rubber spatula just until no dry streaks remain.

Add 3 tablespoons lemon juice to the batter. Whisk briefly until the batter is smooth and evenly combined.

Pour the lemon batter evenly over the candied lemon slices in the cake pan. Smooth the surface gently with a rubber spatula.

Bake at 180°C (350°F) for 30 to 35 minutes. Check doneness by inserting a skewer into the center; it should come out clean.

Cool the cake in its pan for 10 to 15 minutes. Invert it carefully onto a serving plate while still warm.