Line a 9x9-inch metal pan with parchment, leaving an overhang. Lightly spray the parchment. Prepare and bake brownies according to package directions for a 9x9-inch pan. Let cool completely, then lift out and cut into 1-inch cubes.
In a large bowl, whisk together the sweetened condensed milk and 1% milk. While whisking constantly, gradually sprinkle in the pudding mix and continue whisking briskly until smooth. Refrigerate for 5 minutes.In a separate bowl or stand mixer, add heavy cream, powdered sugar, and vanilla. Whisk on low to combine, then increase to high and beat until medium-to-stiff peaks form, about 1 to 3 minutes.Gently fold the whipped cream into the pudding mixture until smooth and combined.
Add one-third of the brownie cubes to the bottom of a trifle dish. Top with one-third of the cream mixture, then about 1 cup berries.Repeat layers two more times: brownies, cream, berries.
Refrigerate for 1 to 3 hours, or overnight for softer brownies. Garnish with fresh mint if desired and serve cold.
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Notes
Note 1: Use one type of berry or a mix such as raspberries, blackberries, and sliced strawberries. I usually add about 1 cup per layer, but adjust to taste.Note 2: Parchment paper helps lift the brownies out cleanly. Wax paper can melt and foil may stick and tear.Storage: Cover tightly and refrigerate leftovers for up to 3 days. For best texture, enjoy within 1-2 days. For make ahead, prepare the layers in advance, then assemble shortly before serving.