Cut the cold butter into small cubes and combine with flour, cornstarch, and salt in a large bowl. Using a pastry cutter or your fingertips, work the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with pea-sized pieces of butter still visible—this creates the tender, flaky texture that makes shortcake special. Stir in the sugar, then add the egg and mix just until a shaggy dough comes together. I prefer not to overmix at this stage, as it can make the shortcakes tough. Wrap the dough in plastic wrap and refrigerate for 20 minutes.

Preheat your oven to 350°F and line two baking trays with parchment paper. On a lightly floured surface, roll out the chilled dough to about 3mm thickness. Using a round cutter (about 2-3 inches in diameter), cut out 18 rounds—you'll need three per serving for layering. Place the cut shortcakes on the prepared trays, spacing them about 1 inch apart. Chill the shaped dough on the trays for another 10 minutes while the oven finishes preheating—this rest helps them hold their shape and bake evenly.

Bake the shortcakes for 15 minutes, or until they're light golden brown on top and firm to the touch. They should smell buttery and smell delicious—that's your cue they're done. Remove from the oven and let them cool on the baking trays for 2-3 minutes before transferring to a wire rack to cool completely. Once cooled, the shortcakes should be crisp on the outside with a tender crumb inside.

While the shortcakes cool, remove the heavy cream from the refrigerator and pour it into a clean, cold bowl—I cannot stress enough how important it is that the cream is very cold, as it's the key to achieving stiff peaks quickly. Using an electric mixer or whisk by hand, whip the cream until soft peaks form, then gradually add the powdered sugar and vanilla while continuing to whip until firm peaks form and the cream looks fluffy and thick. Transfer the whipped cream to a piping bag fitted with a large star tip. While the cream whips, hull and slice the strawberries into 1/4-inch thick rounds, and leave the raspberries whole.

Place a cooled shortcake on a serving plate and pipe a generous dollop of whipped cream from Step 4 on top. Add a few slices of strawberries and several raspberries over the cream. Place a second shortcake on top and repeat the layering with more cream and berries. Top with the third shortcake and finish with a final piping of whipped cream and a handful of fresh berries arranged on top. Serve immediately while the shortcakes are still crisp and the cream is cold.