Heat a medium saucepan, over medium heat, with the half and half and vanilla until steaming, about 3 minutes; remove from heat, cover, and allow to steep 15 minutes (skip this step if using vanilla extract).
Position rack to lower third of oven; preheat to 350 degrees F.
In a small bowl, combine brown sugar and 1/4 teaspoon salt, pressing with spoon to remove any lumps; dump into a 9 inch pie pan and press down in an even layer.
Remove vanilla bean from half and half, scrape out seeds, and return seeds to half and half; heat to steaming, about 1 or 2 minutes.
In a large bowl, whisk together eggs, granulated sugar, and 1/4 teaspoon salt; gradually whisk in half and half mixture until well combined (if using vanilla extract, you would add it here).
Pour mixture into pie plate (do not worry if some of the sugar floats up, it will eventually settle back down to the bottom.
Place pie plate in a deep roasting pan; fill roasting pan with enough water to come halfway up the sides of the pie plate.
Bake for 35 to 40 minutes until a knife inserted in center comes out clean.
Cool flan on wire rack for 30 minutes, cover, and cool in refrigerator at least 6 hours or overnight.
To serve, run a knife around edge and invert on a serving dish.