Cook 1 1/2 cups blueberries, 1/4 cup granulated sugar, and 1 tablespoon lemon juice in a small saucepan over medium heat for 5 to 7 minutes, stirring occasionally, until the berries soften and release their juice.

Mix 1 tablespoon cornstarch with 1 tablespoon cold water. Stir the slurry into the berries and cook for 1 to 2 minutes until thickened, then cool the sauce completely.

Heat 1 cup water, 1/2 cup butter, and 1/4 teaspoon salt in a saucepan until the butter melts and the mixture reaches a full boil. Remove the pan from the heat and add 1 cup flour all at once.

Stir vigorously until a smooth dough forms. Return the pan to medium-low heat and stir for 1 to 2 minutes, until the dough pulls away from the sides and a thin film forms on the bottom.

Transfer the dough to a mixing bowl and cool for 5 minutes. Beat in the 4 eggs one at a time, incorporating each fully before adding the next; stop when the dough is glossy and holds a soft peak.

Heat the oven to 425°F (220°C) and line a baking sheet with parchment. Pipe 12 to 14 mounds about 2 inches wide, leaving space between them, and smooth pointed tips with a damp fingertip.

Bake at 425°F for 10 minutes, then reduce the temperature to 375°F (190°C) and continue baking for 15 to 20 minutes. The shells should be well browned, light, and firm.

Turn off the oven, open the door slightly, and leave the puffs inside for 5 to 10 minutes. Transfer them to a rack to cool completely, piercing each once with a small knife or skewer if you prefer a drier interior.

Beat 1 cup cold heavy cream, 1/4 cup powdered sugar, and 1 teaspoon vanilla until medium-stiff peaks form. Gently fold in about 1/2 cup of the cooled blueberry sauce and reserve the rest for drizzling.

Slice the cooled puffs horizontally or make a small hole in the side or bottom. Pipe in the blueberry cream, dust with powdered sugar, and serve with the reserved sauce and extra blueberries.