In a saucepan, heat 500 ml of full-fat milk until it reaches a simmer. Remove from heat and let it cool down for a few minutes.
In a separate bowl, whisk together 100 grams of sugar and 4 egg yolks until the mixture becomes pale and creamy.
Slowly pour the warm milk into the egg yolk mixture while continuously whisking to avoid curdling.
Transfer the mixture back to the saucepan and cook on low heat, stirring constantly until it thickens and coats the back of a spoon. Remove from heat and let it cool down to room temperature.
In another bowl, mix 200 grams of dark chocolate (chopped) and 100 ml of heavy cream. Melt the chocolate using a double boiler or microwave, stirring occasionally until smooth.
Combine the cooled custard mixture with the melted chocolate and mix well.
Pour the mixture into an ice cream maker and churn according to the manufacturer's instructions. If you don't have an ice cream maker, pour the mixture into a container and freeze for 4-6 hours, stirring every hour to break up ice crystals.
Once the ice cream reaches the desired consistency, transfer it to an airtight container and freeze for at least 2 hours before serving.