To make Key Lime Cheesecake Bars, start by preheating your oven to 375 degrees and line a 9x9 baking pan with parchment paper.
Next, combine graham cracker crumbs, melted butter, dash of salt and powdered sugar together in a mixing bowl then pour the crust mixture into the baking pan lined with parchment paper. Using your fingers or a spoon, firmly press the crust evenly on the bottom of the pan to make your crust. Bake in preheated oven for 8 minutes. Remove and set aside.
While crust is baking, prepare the key limes. Zest 10-12 key limes before cutting all key limes in half. Squeeze your lime juice into a separate small bowl and watch for seeds.
In a large mixing bowl, beat softened cream cheese until creamy and then add in sugar and mix well. Add in eggs and beat until each is incorporated before adding in the next one. Drop in the lime zest and lime juice along with sour cream, salt, vanilla, and continue mixing until you have a creamy filling.
Transfer 1/3 cup of cheesecake filling into a separate bowl and add a few drops of green food coloring. Mix the food coloring into the batter to combine.
Pour the white cheesecake filling on top of the cookie crust crust and smooth until even. Using a spoon, pour lines of green cheesecake filling down the length of the cheesecake. Swirl the colors into the cheesecake filing using the back of knife (or a chopstick!) to create a beautiful swirl pattern being sure not to swirl all the way down the crust. The more swirls, the better..
Bake in oven for 30-35 minutes or until center is no longer too wobbly. Remove the bars from the oven, and cool at room temperature for 30 minutes. Refrigerate until ready to serve.