Melt butter in a medium saucepan over medium heat. Add the pumpkin puree and cook until all the liquid has evaporated, about 6 minutes. Add cream, molasses, and cinnamon and continue to cook, whisking occasionally, until mixture comes just to boiling point, about 5 minutes.
Bring a large pot of water to a simmer. In a stainless-steel bowl, whisk together eggs, yolks, and brown sugar; place the bowl over the pot of water. Slowly add the hot cream mixture and cook, whisking constantly, until it thickens, about 15 minutes.
Pour the mixture into 6 custard cups. Letcool and place in the refrigerator until set, about 3 hours or overnight.
One half hour before serving, heat the broiler. Sprinkle 2 teaspoons of granulated sugar evenly over each of the 6 cups. Place the cups in a pan packed with ice and fill the pan with cold water. Place the pan under the broiler as close to the heat as possible and broil until the sugar turns golden to dark brown, about 3 minutes.