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 a parchment-lined baking sheet to form 8 biscuits. Bake for 15-18 minutes until golden brown. Cool completely, then break into bite-sized pieces.
02 - Heat milk in a saucepan until just simmering. Whisk together sugar, egg yolks, cornstarch, and salt in a bowl until smooth. Gradually whisk hot milk into yolk mixture, then return mixture to saucepan. Cook over medium heat, whisking constantly, until thickened for 3-5 minutes. Remove from heat and stir in vanilla. Pour into a bowl, cover surface directly with plastic wrap, and refrigerate until completely cold.
03 - Combine sliced strawberries, sugar, and lemon juice in a bowl. Toss gently to coat. Let stand for at least 20 minutes at room temperature, stirring occasionally, to release natural juices and soften berries.
04 - Chill a mixing bowl in the freezer for 5 minutes. Add heavy cream, powdered sugar, and vanilla extract to the cold bowl. Beat with an electric mixer or whisk until soft peaks form when beaters are lifted. Do not overbeat.
05 - Layer half the shortcake pieces in the bottom of a trifle dish or large glass bowl. Spread half the custard evenly over the shortcake. Arrange half the macerated strawberries over the custard, including juices. Top with half the whipped cream. Repeat layers with remaining ingredients. Finish with extra strawberries and a dollop of whipped cream if desired.
06 - Refrigerate assembled trifle for at least 1 hour to allow flavors to meld and layers to set. Serve chilled. Store leftovers covered in the refrigerator for up to 2 days.