Step 1

Position an oven rack on the lowest position and preheat the oven to 350 degrees F. Line a 9-by-13-inch baking pan with aluminum foil, leaving overhang two opposite sides (to help remove the bars later) and butter the foil.

Step 2

Combine the graham cracker crumbs and melted butter in a medium bowl. Toss together with a fork to coat evenly, then press into the bottom of the prepared baking pan. Pour the sweetened condensed milk over the top and spread gently with the back of a spoon or an offset spatula so as not to disturb the crust. Arrange 20 of the gelt pieces in a 5-by-4 grid (5 on the long end of the pan, 4 on the short). Sprinkle with the walnuts, then the white chocolate chips, coconut and finally the sprinkles.

Step 3

Bake until set, golden around the edges and the coconut is lightly toasted, 25 to 30 minutes. Remove from the oven and arrange the remaining 20 gelt pieces in another 5-by-4 grid on top, pressing gently. Bake just until the gelt softens and adheres to the coconut, 1 to 2 minutes. Transfer the pan to a wire rack to cool completely, at least 2 hours (see Cook's Note). Remove from the pan using the overhang and place on a cutting board. Cut into 20 bars, so each has a piece of gelt on top.