Heat the oven to 175 C / 350 F. Line a 20 cm / 8-inch square metal baking pan with parchment, leaving an overhang on two sides to lift the bars out later.

Stir 150 g flour, 40 g powdered sugar, 25 g desiccated coconut, and the salt together in a mixing bowl. Rub in the cold butter with your fingertips until the mixture forms clumps that hold when squeezed. Press it into an even layer about 6 mm thick across the pan, using the bottom of a measuring cup, then prick with a fork. Bake for 18 minutes until the edges are light golden.

While the crust bakes, juice the calamansi and strain it through a fine-mesh sieve until you have 80 ml. Whisk the granulated sugar, 30 g flour, and zest together, then whisk in the eggs one at a time. Add the calamansi juice and coconut milk and whisk until smooth with no flour lumps.

Pour the filling over the hot crust and skim off any foam. Bake at the same 175 C / 350 F for 23 minutes, until the edges are set and the center jiggles only slightly when you nudge the pan. Meanwhile, toast the coconut flakes in a dry 20 cm / 8-inch skillet over medium-low heat for 3 to 4 minutes, stirring, until pale gold.

Cool the pan on a rack for 30 minutes, then chill in the fridge, loosely covered, for at least 2 hours until the custard is fully set.

Lift the slab out using the parchment and cut into 16 squares with a sharp knife, wiping the blade clean between cuts. Top with the toasted coconut and dust with 1 tbsp powdered sugar just before serving.