Position the shelf in the center of the oven. Heat the oven to 350 deg F. Cut a 14 inch square of heavy-duty aluminum foil. Invert the pan and center the sheet of foil over the pan, pressing it across the bottom and down the sides. Remove the foil, turn the pan right side up, and place the foil shell in the pan, shaping it smoothly across the bottom and snugly against the sides of the pan. Butter the foil well, then dust with flour, shaking out the excess.

Make the Crust:

Melt the butter in a 2-quart saucepan. Cool to tepid. Add the graham cracker crumbs and the sugar to the melted butter and stir with a fork to combine well.

Sprinkle half of the crumbs (approximately 1 cup) into the pan. Press the crumbs evenly, using a batarde or the bottom of a glass to tamp down the crumbs. Bake for 7 to 8 minutes, or until just set. While the crust is baking, prepare the filling..

Make the Filling:

In the bowl of an electric mixer fitted with the whip attachment, beat the egg yolks on medium-high speed until slightly thickened and lightened in color, 3 to 4 minutes. Slowly add the condensed milk and beat 4 to 5 minutes longer. Reduce the mixer speed to medium-low and mix in the lime juice and zest.

Let the crust rest for 2 to 3 minutes before pouring half of the filling (approximately 1 cup) over it, spreading with the back of a tablespoon. Give the pan several firm taps on a hard surface to spread the filling evenly. Return to the oven and bake for 9 to 10 minutes, until set on top. (It''s okay if a little sticks to your finger.)

Sprinkle the remaining crumbs over the warm filling, spreading them carefully. Press gently with the bottom of a glass to even the crumbs. Bake for 8 to 9 minutes, or until set. Let rest for 2 to 3 minutes.

Pour the remaining filling mixture over the crust, spreading evenly with the back of a tablespoon. Tap the pan on the counter to even the filling. Bake for 10 minutes, or until set.

Let stand for 2 to 3 minutes, then sprinkle with approximately half of the toasted coconut, pressing it in gently. Then top with the remaining coconut. Refrigerate for 4 to 5 hours before removing the pan. Using a long, thin-bladed knife dipped in warm water as needed and wiping clean with a paper towel, cut into 25 squares.