01 - In a medium bowl, combine strawberries, raspberries, blueberries, granulated sugar (if using), and lemon juice. Toss gently and let sit for 10 minutes to macerate.
02 - In a large bowl, whip the heavy cream with powdered sugar and vanilla extract until soft peaks form.
03 - To assemble, layer one-third of the cake cubes into the bottom of each of 6 clear cups or glasses.
04 - Spoon a layer of whipped cream over the cake, followed by a layer of mixed berries.
05 - Repeat the layers (cake, cream, berries) until the cups are full, finishing with a layer of whipped cream and a few berries on top.
06 - Garnish with fresh mint leaves if desired. Serve immediately, or chill in the refrigerator for up to 2 hours before serving.