This souffle has a fruit-puree base, which makes it very light and intensely flavored.
Preheat oven to 400 degrees. Butter dishes well, coat with sugar, and tap out excess. Chill.
Peel, quarter, and core pears. Cut into 1-inch pieces. Squeeze lemon halves over pieces to prevent discoloring. Place pears and 2 tablespoons sugar in a small saucepan and cook, stirring frequently, over medium heat until they become a chunky sauce, about 10 minutes. Transfer to a food processor, and puree. Return to pan.
Using a clean garlic press, squeeze juice from ginger into puree; discard pulp. Continue cooking puree over low heat until thick and a spoon drawn through puree leaves a trail, about 15 minutes.
In a small bowl, dissolve the cornstarch in 2 tablespoons cold water. Stir into the puree, and bring to a boil, stirring constantly. Simmer a few minutes more. Transfer to a large bowl, and beat in egg yolks. Cover with plastic wrap, pressing it onto the surface of the mixture. Keep warm until ready to use.
In a large bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add remaining 3 tablespoons sugar. Beat until stiff and glossy.
Spoon 1/3 of the whites into pear base. Add Poire William, if desired, and whisk until smooth. Quickly fold in remaining whites until combined. (Don't worry if there are some streaks.) Fill prepared dishes to about 1/4 inch from the top. Run your thumb around edge of dishes to make a groove. Sprinkle with granulated sugar if desired.
Bake 10 minutes at 400 degrees, then reduce heat to 375 degrees and continue baking for 10 minutes. Serve immediately.