Slice the mangoes into thin, wide pieces, ensuring they're ripe but firm enough to maintain their shape. Try to make the slices consistent in thickness for even layering throughout the cake.

Whip the cold heavy cream in a large bowl using an electric mixer until it reaches stiff peaks. Gently fold in the sweetened condensed milk until fully incorporated, being careful not to deflate the whipped cream. This creates a stable, sweet filling that will set properly in the refrigerator.

Line the bottom of a rectangular baking dish or mold with a single layer of graham crackers, breaking pieces if needed to completely cover the surface. This will form the sturdy base of your refrigerator cake.

Spread an even layer of the cream mixture over the crackers, about 1/2 inch thick. Place mango slices in a single layer on top of the cream, covering as much area as possible for maximum flavor in every bite.

Repeat the layering process in the order of crackers, cream, and mango slices until you've used most of your ingredients, aiming for at least 3 complete layers for the best texture and appearance.

Cover the final layer with a generous amount of cream, creating a smooth, even surface. Sprinkle biscuit crumbs evenly across the top and arrange a few decorative mango slices for an attractive presentation.

Cover the dish with plastic wrap and refrigerate for at least 4-5 hours, preferably overnight. This resting time allows the crackers to soften from the moisture in the cream and transforms the dessert into a cake-like consistency.