Rinse the 4 lemons under running water and dry them with a kitchen towel.

Place the lemons on a chopping board and carefully cut each lemon in half.

Use a spoon to scoop out the lemon pulp, trying to keep the lemon shells intact. You can set the pulp aside for other uses (e.g., making lemon water). Set the lemon shells aside for later use.

In a saucepan, add 125g of milk, 125g of heavy cream, and 15g of caster sugar. Heat over low heat, stirring constantly to prevent the mixture from burning.

Grate half a lemon's zest using a fine grater, being careful not to grate the white pith, which may cause bitterness. Add the lemon zest to the saucepan, then add a small amount of vanilla paste. Continue stirring until the sugar completely dissolves.

Once the sugar is dissolved, remove the saucepan from the heat. Stir the mixture to help it cool down until it reaches a warm temperature, about 40-50°C (not too hot to the touch).

In a small bowl, add enough cold water to soak the 4g of gelatin sheets. Let them soak for 10 minutes, or until softened.

Once the gelatin sheets are soft, remove them carefully using your hands or tongs and squeeze out the excess water. Add the softened gelatin sheets to the warm milk mixture and stir until the gelatin is fully dissolved.

Carefully pour the milk jelly mixture into the prepared lemon shells, filling each shell about 80% full. Be careful not to overfill, to prevent spillage.

Gently place the filled lemon shells into the refrigerator and chill for at least 3 hours. The jelly will only set properly after sufficient time in the fridge; if chilled for too little time, the jelly may remain too soft and not firm up.

After the jelly has set, remove the lemon shells from the refrigerator. Use a fine grater to add a bit more lemon zest on top of the jelly for decoration, enhancing both the fragrance and visual appeal.

You can scoop the jelly directly with a spoon and enjoy the refreshing, tangy, and sweet lemon-flavored milk jelly. If you don't finish it all, cover the remaining jelly with plastic wrap and return it to the fridge. However, it's best consumed as soon as possible.