Prepare the pie plate by spraying it with non-stick spray and line the bottom with parchment paper as it makes it easy to remove the pie slices
Crumb the graham crackers (I put them in a zip-block bag and crush them with a pestle) and place them in a mixing bowl
Melt the butter in a heavy bottom sauce pan over medium heat. Regularly stir with a spatula. Heat till the butter melts, then some foam will form at the surface. The butter liquid has evaporated and the milk solids are caramelizing creating a brown color. Check the butter color through the foam by stirring, until it reaches a dark brown color. Don't walk away during this process as the butter could burn in an instant at this stage. Remove from heat.
Pour the brown butter, including the dark bits at the bottom of the pan, into the crumbs in the mixing bowl. Mix well.
Scoop 2/3 of the crumbs into the prepared pie plate and reserve the remaining third. With your fingers, press down the crumb mixture to get an even and firm crust at the bottom and around the edges (about 1/2 inch or 1.3 cm height). Put the crust in the freezer to firm up.
To prepare the cream, pour the condensed milk in a medium size whipping bowl and add the heavy whipping cream. Beat the mixture with a hand mixer set at medium low speed until thickened and peaks form. To test, lift the whisk and the cream should not dribble. Be careful not to over whip or the cream will solidify into butter. Make sure to use medium low speed to achieve a creamy and stable texture after freezing. High speed whipping would incorporate too much air too fast and the cream will be very fluffy but will collapse.
Cube the mangoes. With a gloved hand, squish handful of mangoes to get a mixture of pureed and whole mango cubes. If you wish, add the lime juice for some acidity balance.
To assemble the pie: Remove the crust from the freezer, add a layer of cream (about half of the cream). Add about half of the mango cubes and sprinkle half of the remaining crumbs. Add another layer by repeating the same process, add cream, mango and crumbs
Wrap the pie with shrink wrap and freeze overnight or for at least 8 hours
To serve, remove the pie from the freezer an hour before serving, or defrost in the refrigerator for 3-4 hours. The pie is best served semi-frozen.