Sift the pastry flour and cornstarch together into a small bowl and set aside. Line a shallow baking sheet or quarter-sheet pan with premium plastic wrap.
Slice the vanilla bean lengthwise and scrape out the seeds. Add both the seeds and the empty pod into a medium, heavy-bottomed saucepan with the whole milk. Heat over medium until it reaches a gentle simmer, then remove from heat to steep.
In a large mixing bowl, combine the egg yolks and granulated sugar. Whisk immediately and vigorously until the mixture becomes pale yellow, thick, and falls in ribbons.
(Pro-tip: Never let sugar sit unmixed on yolks, or it will curdle them into hard, irreversible lumps).
Add the sifted flour and cornstarch to the egg yolk mixture. Whisk gently only until just incorporated to prevent over-activating the gluten.
Discard the vanilla bean pod from the warm milk. While whisking the egg mixture continuously with one hand, slowly pour a steady stream of about one-third of the warm milk into the bowl to temper the yolks. Once completely smooth, whisk in the remaining milk.
Pour the entire loose mixture back into the saucepan. Place over medium heat and whisk rapidly and continuously, ensuring you reach all the way into the bottom seams and corner edges of the pan to prevent scorching.
Place over medium heat and whisk rapidly and continuously, ensuring you reach all the way into the bottom seams and corner edges of the pan to prevent scorching.
As the mixture thickens dramatically, keep whisking hard. Once it transitions from a dull paste to a beautiful, smooth, glossy cream and large bubbles break the surface, continue cooking for exactly 30 to 45 seconds. This fully cooks the starch and eliminates raw flour flavors.
Immediately remove from heat and pour the hot cream onto your prepared shallow pan. Smooth it out flat. Press a sheet of plastic wrap directly onto the surface of the cream (touch-top) to prevent a skin from forming. Chill in the refrigerator until completely cold (at least 2 hours).
Once thoroughly chilled, the cream will have a firm, jelly-like set. Transfer it to a clean bowl and beat vigorously with a flexible spatula or a stand mixer fitted with a paddle attachment until it completely softens into a luxurious, silky, pipeable elegance.
(Tips: For an absolutely flawless, professional outcome, pass the loosened cream through a fine-mesh strainer one final time using a spatula. This extra polish breaks down any microscopic graininess and ensures the ultimate smooth and silky mouthfeel.)