In a bowl, combine butter, flour, sugar and salt. With hands, mix together until just combined. Transfer dough to a ungreased skillet and gently press into bottom of skillet. Using the back of a spoon, gently smooth and even out surface of dough. Bake in a 350 F oven for about 15 to 20 minutes or until firm and lightly browned. Remove from oven and allow to cool.

In a bowl, whisk together sugar and flour. Add eggs and calamansi juice. Whisk together until well blended. Pour over crust and bake in a 350 F oven for about 20 to 25 minutes or until set. Remove from pan and allow to cool. Using a fine mesh sieve, dust top with powdered sugar. Slice into servings.