Prepare the Pastry Cream: In a large bowl, whisk together the 3 eggs, extra yolk, and granulated sugar until pale and thick, about 2 minutes. Gradually add cornstarch, flour, and salt, whisking until smooth, then set aside.

Heat the Milk: Warm the whole milk in a saucepan over medium heat until steaming, but do not boil. Slowly incorporate about ¼ cup of hot milk into the egg mixture while whisking, then stir in the remaining milk.

Cook the Custard: Return the custard mixture to the saucepan and whisk continuously over medium-low heat for 7–9 minutes, or until thickened to pudding-like consistency.

Chill the Pastry Cream: Transfer to a bowl, stir in vanilla extract, cover with plastic wrap, and refrigerate for 1–2 hours to cool completely.

Make the Choux Pastry: Preheat oven to 425°F (220°C). Combine unsalted butter, water, sugar, and salt in a saucepan, simmer until butter melts, then remove from heat.

Combine Flour and Dough: Quickly add flour to the warm butter mixture, stirring vigorously. Return to low heat for 1–2 minutes until the dough forms a ball.

Incorporate the Eggs: Allow dough to cool slightly, then add eggs one at a time, mixing well after each addition until the dough is smooth and glossy.

Prepare for Baking: Trace a 9-inch circle on parchment paper, flip it, and pipe or scoop mounds of dough along the circle.

Bake the Choux Pastry: Bake for 10 minutes at 425°F, then reduce to 375°F and continue for 20–25 minutes until deep golden-brown.

Cool the Pastry: Pierce each mound with a toothpick to release steam, turn off the oven, and leave the pastry inside for 15 minutes to dry.

Whip the Cream: Whisk heavy cream until thickened, add powdered sugar and vanilla, and beat until soft peaks form.

Assemble Your Paris–Brest: Slice the pastry ring in half horizontally, fill the bottom with pastry cream, top with whipped cream, and replace the upper half. Dust with powdered sugar.