Pour 100 ml of the cold milk into a medium bowl. Add the cornstarch and whisk until completely smooth with no lumps. Add the egg yolks and whisk again until combined. Set aside.

Combine the remaining 500 ml milk, dark brown sugar, and a pinch of salt in a 2-quart saucepan. Heat over medium, stirring occasionally, until the sugar dissolves and the milk is hot but not boiling, about 4 minutes.

Slowly ladle about 80 ml of the hot milk into the cornstarch-yolk mixture while whisking constantly. This tempers the eggs so they don't scramble. Whisk until smooth.

Pour the tempered mixture back into the saucepan in a slow stream, whisking the pot continuously. Reduce heat to medium-low.

Cook, stirring constantly with a wooden spoon or silicone spatula, for 5 to 8 minutes until the pudding thickens enough to coat the spoon and a drawn line holds for 3 seconds. Do not let it boil hard.

Remove from heat and stir in the vanilla extract. Pass through a fine-mesh sieve into a pourable jug to catch any lumps.

Divide the pudding evenly between 4 ramekins or serving cups. Press a sheet of plastic wrap directly onto the surface of each to prevent a skin from forming.

Refrigerate for at least 3 hours, or overnight, until firm and set. Serve chilled, directly in the ramekin or turned out onto a small plate.