Heat oven to 375°F and arrange rack in the middle.

If you are using whole graham crackers, break them into small pieces and place them in the bowl of a food processor fitted with the blade attachment. Pulse until crackers are broken into crumbs. Add sugar and butter and pulse again until well mixed. If you are using store bought crumbs, just mix all the ingredients together in a bowl. If you don't have a food processor you can place the whole graham crackers in a resealable plastic bag and bash them with a rolling pin until they are broken into crumbs, pour the crumbs into a bowl and mix with the sugar and butter.

Transfer mixture to a 9-inch pie plate and evenly press into the plate with your fingers. Place the crust on a baking sheet and bake for 15 minutes or until it starts to brown on the edges. Remove from oven and let cool completely before filling.

Lower oven temperature to 325°F.

Combine sweetened condensed milk, lime juice, eggs, and lime zest in a medium bowl and whisk until smooth.

Pour into the cooled crust and bake for 15 minutes. Remove from oven and let cool. Chill in the refrigerator for at least 2 hours before serving.

When ready to serve, combine sour cream and powdered sugar in a bowl. Spread evenly over the pie with a spatula, slice and serve.