Line a 7" x 11" pan with parchment paper. Set aside.
In the chilled bowl of a stand mixer and balloon whisk attachment, beat together the chilled heavy cream and chilled sweetened condensed milk until stiff peaks form, about 5-6 minutes.
Add the vanilla extract and kosher salt. Beat for 1-2 more minutes, until combined.
Place a layer of graham crackers across the entire bottom of the lined pan, filling in any gaps with pieces of graham cracker.
Scoop out 1/3 of the whipped cream mixture and smooth out into an even layer on top of the graham crackers, spreading out to the sides of the pan.
Spoon 1/2 of the diced mangoes and sprinkle over the whipped cream mixture.
Repeat layers: graham crackers, 1/3 of the whipped cream mixture, then remaining half of the diced mangoes.
For the final top layer, lay out graham crackers, remaining 1/3 of the whipped cream mixture, then place sliced mangoes on top.
If desired: sprinkle crushed graham crackers on top.
Carefully cover the top and chill in the refrigerator for at least 2 hours.
Then move to the freezer for at least 6 hours or overnight.
Thaw at room temperature for 5-10 minutes before slicing and serving.
Notes
Makes approximately 8-12 servings.Freeze: Store in the freezer in an airtight container for up to one month. Mangoes: if you can, use Alfonso or Ataulfo mangoes. These are both sweeter and creamier than other mango varieties. I can usually find Ataulfo mangoes at Costco and H-Mart (check your local Asian markets). Pick mangoes that give slightly when you squeeze them. If they are firm all around, they're likely not ripe enough.