In a saucepan, heat 500 ml of full-fat milk until it reaches a simmering point. Do not let it boil.
In a separate bowl, whisk together 150 grams of granulated sugar and 4 egg yolks until the mixture becomes pale and creamy.
Gradually pour the hot milk into the egg yolk mixture, whisking continuously 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. This should take about 10-15 minutes.
Remove the saucepan from heat and let it cool to room temperature. Stir in 1 tablespoon of pure vanilla extract.
In a separate bowl, whip 250 ml of heavy cream until soft peaks form. Gently fold the whipped cream into the cooled custard mixture.
Pour the mixture into an airtight container or an ice cream maker, and freeze for at least 6 hours or overnight, stirring every hour to break up any ice crystals.