Whisk together pumpkin and vanilla and chill at least 3 hours and up to 8 hours. In a heavy 2 quart saucepan, combine 1/2 cup of the cream and 1/2 cup of brown sugar. Cook over medium heat until bubbles form around the edges of the pan, about 5 minutes. Meanwhile, combine yolks with spices, remaining cream and brown sugar. Whisk and smooth until the sugar begins to dissolve. Remove cream mix from heat and gradually whisk about 1/2 cup of the hot cream into the egg mixture, whisking constantly until smooth. Pour into the saucepan and cook over medium heat, stirring constantly with a wooden spoon. Keep the custard at a low simmer until it is thick enough to coat the back of a spoon and leaves a clear trail when a finger is drawn through it, approximately 4-6 minutes. Do not let the custard boil. Strain through a fine mesh sieve and into a bowl. Place bowl in an ice water bath. Stir occasionally to release steam until cool. Whisk pumpkin mixture into custard and cover with plastic wrap that lays directly on the surface of the custard (this will prevent a “skin” from forming). Chill at least 3 hours and up to 24 hours. The longer it sits, the better the flavor. Pour custard into an ice cream maker and follow manufacturer’s instructions. Add bourbon or pie spice during the last minute of freezing. Cover and freeze for at least 3 hours and up to 3 days before serving.