Preheat oven to 350F. Line an 8x8-inch metal baking pan with parchment paper. Set aside.
To make the crust: Add the graham cracker crumbs, brown sugar, and butter to a medium mixing bowl.
Stir to combine.
Pour the crumb mixture into the prepared pan.
Press the crumbs to form a firm, even layer in the bottom of the pan. See the note below for tips.
Bake the crust for about 10 minutes, or until the edges are just beginning to turn light golden brown.
While the crust is baking prepare the filling.
To make the filling: Add the cream cheese to a large bowl.
Beat the cream cheese until it's smooth and well-beaten.
Add the sweetened condensed milk and mix on until well-combined.
Stir in the eggs until well-combined, scraping down the bowl as-needed.
Add the lime juice, lime zest, and vanilla extract. Mix in until combined.
When the crust has finished baking, pour the filling over the hot crust.
Return to the oven.
Bake for 18-25 minutes, or until the filling is set. You don't want the filling to brown.
Remove from the oven, and place on a wire cooling rack.
Let cool to room temperature.
Once at room temperature, chill for at least 2 hours before serving.
Cut into bars to serve.
Store, covered, in the refrigerator.
Graham cracker crumbs: Prepared graham cracker crumbs (from a box) work well in this crust. If you'd like to make your own crumbs, you'll need about 9-10 full sheets of regular graham crackers.
Brown sugar: Light or dark brown sugar will work. I don't firmly press the brown sugar, but you can if you want a sweeter crust.
Butter: I use salted butter. If you use unsalted butter, add ¼ teaspoon of salt to the crumb mixture.
Cream cheese: I use full-fat cream cheese in this recipe.
Sweetened condensed milk: I've only used full-fat sweetened condensed milk in this recipe.
Key lime juice: Feel free to use fresh or a good-quality bottled Key lime juice. If you use bottled Key lime juice, you will still need a lime for zest. If you can't find Key limes, Persian limes (usually sold as "limes") can be used as a substitute. I don't recommend using bottled lime juice.
Pressing the crust: I start by patting the crust down and into place with a rubber or silicone spatula. Once it's in place, I use the bottom of a measuring cup to pack the crust down.