In a medium skillet, melt 1 tablespoon of butter until foaming. Add the sage and cook over moderately high heat, stirring, until crisp, about 30 seconds. Transfer to a paper towel–lined plate. Melt another 1 tablespoon of the butter in the skillet until foaming. Add the shallot and cook over moderately high heat until translucent, about 1 minute. Add the pumpkin puree, salt, pepper and 1/2 cup of the heavy cream and stir until very thick and reduced to 1 cup, about 4 minutes. Scrape the pumpkin mixture into a bowl and stir in the remaining 1/2 cup of heavy cream and the milk. Let cool to room temperature, then refrigerate until chilled, about 1 hour.