In a saucepan, combine the 1/2 cup of sugar and the water to make a caramel. Once it reaches a golden amber color, pour it into the bottom of the ramekins.
In another saucepan, add the milk and heavy cream.
Split the vanilla bean in half, scrape out the seeds, and add both the seeds and the pod to the saucepan.
Finely grate a small amount of tonka bean into the mixture to give the custard a subtle caramel-like flavor.
Bring the mixture to a boil, then remove from the heat and let it infuse.
In a mixing bowl, whisk together the eggs and the 70 g of sugar until slightly pale and smooth. Gradually pour in the warm milk and cream mixture while whisking, then divide the custard evenly among the ramekins.
Place the ramekins in a baking dish and pour hot water around them until it reaches halfway up the sides (water bath).
Cover the ramekins with a silicone baking mat (or heatproof covering) to prevent a skin from forming, then bake for about 20 minutes.
Allow the custards to cool in the turned-off oven, then refrigerate overnight before serving