01 - Whip the chilled heavy cream in a mixing bowl using an electric mixer on medium-high speed until soft peaks form. Gently fold in the sweetened condensed milk until fully incorporated and smooth.
02 - Arrange a single layer of graham crackers in the bottom of a 9x13-inch glass dish, breaking crackers as needed to fit the space completely.
03 - Spread one-third of the cream mixture evenly over the graham cracker layer using a spatula.
04 - Distribute one-third of the sliced mangoes evenly over the cream layer.
05 - Repeat the layering process two more times: graham crackers, cream mixture, and sliced mangoes. Finish with a final layer of mangoes on top.
06 - Sprinkle reserved graham cracker crumbs over the top if desired. Cover tightly with plastic wrap and refrigerate for at least 6 hours or overnight until completely set.
07 - Slice into squares and serve chilled. Garnish with fresh mint leaves if using.