01 - Preheat oven to 400°F. Whisk together flour, sugar, baking powder, and salt in a large bowl. Cut in cold butter until mixture resembles coarse crumbs. Whisk milk and egg in a small bowl, then add to flour mixture. Stir until just combined. Drop large spoonfuls onto parchment-lined baking sheet to form 8 biscuits. Bake 15–18 minutes until golden. Cool completely, then break into bite-sized pieces.
02 - Heat milk in a saucepan until just simmering. Whisk sugar, egg yolks, cornstarch, and salt until smooth in a separate bowl. Gradually whisk hot milk into yolk mixture, then return to saucepan. Cook over medium heat, whisking constantly, until thickened, about 3–5 minutes. Remove from heat and stir in vanilla. Pour into a bowl, cover surface with plastic wrap, and chill until cold.
03 - Toss sliced strawberries with sugar and lemon juice in a bowl. Let sit for at least 20 minutes to release juices.
04 - In a cold bowl, whip heavy cream with powdered sugar and vanilla until soft peaks form.
05 - Layer half the shortcake pieces in a large trifle dish or glass bowl, followed by half the custard, half the strawberries with juices, and half the whipped cream. Repeat layers. Finish with extra strawberries and whipped cream if desired. Chill at least 1 hour before serving.