Line an 8 x 8 baking dish with foil, leaving about a 1-inch overhang on sides. Preheat oven to 350 F.
In a bowl, combine graham cracker crumbs, flour, brown sugar, and salt. Stir until well-dispersed.
Add butter and using two knives or pastry cutter, cut butter into the flour until the mixture resembles coarse crumbs.
Transfer graham cracker mixture into the prepared pan. Using hands, press crumbs onto the bottom of the pan. Use a flat-bottomed glass to press the mixture evenly and to ensure the crust is well compacted.
Bake for about 8 to 10 minutes.
In a large bowl, combine cream cheese and sugar. Using a hand mixer, beat until smooth and creamy.
Add eggs, beating at low speed after each addition.
Add flour and stir well.
Add calamansi juice and baking powder. Stir until well-combined.
Pour over the crust and spread evenly with a spatula.
Bake for about 25 minutes or until the center is set.
Remove from heat and allow to cool for about 1 hour. Refrigerate for at least 2 hours or overnight until completely chilled and set.
Dust top with powdered sugar just before serving and cut into squares.