Preheat the oven to 350° F. Line a 9x9 glass pan with parchment paper and set aside.

Crush the graham crackers until they are a fine dust. You can either use a food processor, or put them in a ziploc bag and crush them with a rolling pin. Move the graham cracker crumbs to a bowl and mix in the granulated sugar. Then mix in the melted butter.

Gently press the graham cracker crust into the lined 9x9 pan with the back of a measuring cup until it's even and there aren't any loose crumbs. Bake for 10 minutes. Remove the crust from the oven, but leave the oven on.

In a medium-sized bowl, beat the egg yolks with a hand mixer for about 2 minutes on high speed. Add the sweetened condensed milk, and beat on high for another 2 minutes. Then add the key lime juice and lime zest, and mix on low until combined.

Pour the key lime filling over the top of the crust and smooth over with a spatula or knife. Bake for about 15 minutes, or until the center is set.

Remove the key lime pie bars from the oven and allow to cool on the counter for about 30 minutes. Loosely cover with plastic wrap and let the bars sit in the fridge overnight (or at least 4 hours). Serve cold. Enjoy!