Add the sugar and work it until it becomes a smooth cream.
Add the whipping cream slowly, stirring constantly.
Add the eggs one by one, mixing them well and then add the melted butter, continuing to mix
Add the lemon zest and the sifted flour and work the mixture until smooth.
Pour the mixture into the molds (I used the colorful silicone ones bought from Amazon, they are extraordinary) put a bit of chocolate (as big as the top part of your index finger) into the batter and push it down so when it melts you have a chocolate layer there.
Bake at 180°C (350°F) in a preheated oven for at least 50 minutes. They will be ready when their insides are cooked testing them with a toothpick.