Preheat the oven to 350 degrees.
Put the granulated sugar in a 9-inch cake pan or pie plate, set on the center rack in the oven, and bake until the sugar is caramel colored, 8 to 12 minutes. Swirl to cover the bottom of the pie plate with the caramel.
In a large bowl, whisk together the brown sugar, cinnamon, ginger, nutmeg, and salt. Stir in pumpkin puree. In a medium bowl, whisk together the half-and-half, eggs, and vanilla. Thoroughly blend the egg mixture into the pumpkin puree.
Set the pie plate in a large roasting pan, and pour the custard over the caramel. Carefully pour enough hot tap water into the roasting pan to reach halfway up the sides of the pie plate. Bake until the custard is set, about 1 hour and 10 minutes. Cool and chill in the refrigerator. Run a knife around the outside edge of the flan and invert it onto a rimmed plate. Cut into wedges or scoop and serve with a dollop of whipped cream.