Step 1: In a large, chilled mixing bowl, combine the very cold heavy cream, chilled sweetened condensed milk, and optional vanilla extract. Using an electric mixer, whip the mixture on medium-high speed until firm peaks form. Be careful not to over-whip.
Step 2: Prepare your baking dish (an 8x8 or 9x13 inch dish works well). Arrange a single layer of graham crackers at the bottom of the dish, breaking them as needed to fit. You can also crush some for a bottom layer if preferred.
Step 3: Spread about one-third of the whipped cream mixture evenly over the graham cracker layer.
Step 4: Arrange a layer of thinly sliced mangoes over the cream, covering it as much as possible.
Step 5: Repeat the layering process: add another layer of graham crackers, followed by another third of the cream mixture, and then another layer of mangoes.
Step 6: Place the final layer of graham crackers, then top with the remaining cream mixture. Arrange any remaining mango slices decoratively on top.
Step 7: Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the crackers to soften and the dessert to set completely.
Step 8: Once chilled, slice and serve your delicious No-Bake Mango Float. Enjoy your tropical treat!