Make the custard: Whisk together 1/2 cup sugar, cornstarch, and egg yolks in a medium bowl until smooth. Heat milk and remaining 1/4 cup sugar in a saucepan over medium heat until steaming but not boiling.

Slowly pour half the hot milk into the egg mixture while whisking constantly to temper the eggs. Pour everything back into the saucepan and cook over medium heat, whisking constantly, until thickened and bubbling, about 3-4 minutes.

Remove from heat and whisk in vanilla and butter until smooth. Transfer custard to a shallow dish, press plastic wrap directly onto the surface, and refrigerate until completely cold, at least 2 hours.

Toss all the berries with 2 tablespoons granulated sugar in a bowl and let macerate for 15 minutes. Drain excess juice if very liquidy.

Whip the heavy cream with powdered sugar in a large bowl until soft peaks form, about 2-3 minutes. Gently fold half the whipped cream into the chilled custard until combined.

Assemble the trifle: Spread one-third of the cake cubes in the bottom of a 3-quart trifle dish or glass bowl. Drizzle with liqueur if using. Layer with one-third of the custard cream, then one-third of the berries.

Repeat layers two more times, ending with berries on top. Pipe or dollop the remaining plain whipped cream over the final berry layer.

Cover and refrigerate for at least 4 hours or overnight. The flavors will meld and the cake will soften to the perfect texture. Garnish with fresh mint or extra berries just before serving.