Using a slotted spoon, transfer pears to a food mill and process until dry skins are all that remain. Scrap the bottom of the food mill as a lot of the puree will stick. Discard skins and transfer puree back into the Dutch oven. Stir in sugars, lemon juice and salt. Simmer over low heat (otherwise, it will splatter!), stirring occasionally, until mixture is browned and thickened and rubber spatula or wooden spoon leaves distinct trail when dragged across bottom of pot, about 45 minutes to 1 hour. Transfer pear butter to a jar with tight-fitting lid and let cool to room temperature before refrigerating. Pear butter can be refrigerated for up to 1 month.