Crust

1. Heat oven to 325ºF and adjust rack to middle position. Line an 8 inch baking pan with foil, hanging the foil over the sides so the bars will be easily removable. Spray with vegetable or canola oil.
2. Pulse graham crackers in food processor until broken down into a fine even crumb, you should have about 1 1/4 cups. Add brown sugar and salt and pulse together. Drizzle melted butter over the crumbs and pulse until crumbs are evenly moistened.
3. Press the crumbs evenly and firmly into the bottom of the prepared pan.
4. Bake until fragrant and golden brown, 18-20 minutes. Cool on wire rack while making the filling.

Filling

1. In a medium bowl with a rubber spatula, stir together the cream cheese, zest, and salt until softened and thoroughly combined. Add sweetened condensed milk and whisk until incorporated and no lumps of cream cheese remain. Whisk in egg yolk, then gently whisk in lime juice until the mixture is fully incorporated.
2. Pour the filling into the crust and spread to the corners. Smooth the surface with a rubber spatula. Bake 15-20 minutes or until set and the edges begin to pull away slightly from the sides. Cool on wire rack to room temperature. Cover and refrigerate until chilled, at least 2 hours.
3. Loosen edges with small knife and lift bars out of pan using the foil extensions. Cut bars into 16 squares. Sprinkle with toasted coconut if desired and serve.