Peel and core pears, and cut into 1/2- inch pieces. Place in a steamer rack set over a pot of boiling water, and cook until soft, about 15 minutes. Let cool. Puree in a blender until completely smooth.
Whisk egg yolks, sugar, and salt in a medium bowl. Whisk in cornstarch. Heat milk in a medium saucepan over medium- high heat just until bubbles begin to form, about 3 minutes. Using a ladle, gradually add hot milk to the yolk mixture, whisking constantly. Return mixture to pan; cook over medium heat, whisking constantly, until mixture begins to bubble and becomes very thick, about 8 minutes. Using a wooden spoon, scrape bottom and sides of pan occasionally as mixture thickens. Remove from heat.
Pour through a large-mesh sieve into a bowl. Add butter; stir until butter has melted. Add pear puree; stir to combine. Place plastic wrap directly on surface; refrigerate until cold, about 30 minutes.
Put heavy cream into the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form. Fold whipped cream into pastry cream.
Transfer to a pastry bag fitted with a 1/4-inch plain round tip (such as Ateco #802). Poke a small hole in the bottom of each pastry puff with a skewer. Fill puffs with pastry cream. Arrange 4 cream puffs on each plate in a pyramid. Spoon about 4 warm caramel pear slices over each serving, and drizzle with sauce.