Adjust oven rack to middle position and heat oven to 350 degrees. Prepare a cookie sheet with parchment paper or silicone baking mat, and arrange 1 1/2 cups coconut in a single layer and toast in the preheated oven for 8-10 minutes.
Line a 9x13 pan with parchment paper.
Process flour, sugar, cornstarch, and salt in a food processor until combined 15 seconds.
Add butter and process to blend 8 to 10 seconds, then pulse until the mixture is pale yellow and resembles coarse meal about 3 pulses.
Add in 3/4 cup of the toasted coconut and pulse 2-3 more times. If doing this by hand, use a pastry cutter to cut the butter into flour mixture.
Sprinkle the mixture into the prepared pan and press firmly with your fingers into an even 1/4-inch layer over the entire pan bottom about 1/2-inch up the sides.
Refrigerate for 30 minutes then bake until golden brown, about 20 minutes.