Half-fill a large bowl with ice water; set aside. Whisk ½ cup sugar, cornstarch and salt in a medium heatproof bowl. Whisk in egg yolks until the mixture is pale yellow, smooth and fluffy, about 1 minute.

Combine milk and lemon zest in a medium saucepan; bring to a gentle simmer over medium heat, stirring occasionally. Remove from heat. Slowly add the warm milk mixture to the egg yolk mixture, whisking constantly, until all of the milk mixture has been incorporated. Return the mixture to the saucepan. Cook over medium heat, whisking constantly, until the mixture begins to bubble and has thickened, about 3 minutes. Remove from heat; whisk in butter until combined. Pour through a fine-mesh sieve into a clean medium heatproof bowl. Immediately place plastic wrap or buttered parchment paper directly on the surface of the pastry cream to prevent a skin from forming. Place the bowl in the prepared ice bath; let cool for 30 minutes. Remove from the ice bath and refrigerate until chilled, about 2 hours.

Stir 2 tablespoons lemon juice into the chilled pastry cream; set aside. Stir blueberries and the remaining 2 tablespoons sugar together in a large bowl, mashing some berries to release juices; let stand, uncovered, at room temperature, until the berries are glossy, about 5 minutes.

Gently toss cake, simple syrup and the remaining 3 tablespoons lemon juice together in a large bowl until coated. Place half of the cake in an even layer in the bottom of a small (2-quart) trifle bowl or medium glass bowl. Spoon half of the pastry cream over the cake, smoothing into an even layer. Spoon half of the blueberries with juices in an even layer over the pastry cream. Repeat the layers with the remaining cake, pastry cream and blueberries. Garnish with lemon zest, if desired.