Whisk the sugar, eggs, half-and-half, cinnamon, allspice, salt, pumpkin puree, and sour cream in a large bowl. Pour the mixture into 6 ounce ramekins set inside a baking pan.
Carefully pour boiling water halfway up the outsides of the ramekins, making sure to not get water into the custard. Very carefully place the pan into the oven. Bake for 25 minutes or until a knife inserted into the center of the flan comes out clean.
Remove the ramekins from the water and place on a cooling rack to cool. Then chill once they reach room temperature until chilled (about 4 hours).
Run a thin metal spatula around the inside of the ramekins and place the ramekin into hot water for a few minutes to loosen the caramel.