Preheat the oven to 340°F (170°C). In a saucepan, heat 2/3 cup sugar with 1 tablespoon water to make caramel. Cook until it turns brown. Remove from heat and pour a little caramel into the bottom of each ramekin.

In another saucepan, heat the milk, heavy cream, and vanilla bean. Grate a little tonka bean into the mixture, bring to a boil, then let it infuse. Meanwhile, in a mixing bowl, whisk the eggs with 1/3 cup sugar. Pour the hot milk and cream mixture over the eggs while whisking. Pour the custard into the ramekins over the caramel and bake for 20 minutes.