Preheat the oven to 350°F. Grease an 8x8-inch pan with butter or nonstick spray. Line the pan with parchment paper, leaving a 2-inch overhang on at least two sides.

In a large bowl, combine the graham cracker crumbs and sugar. Drizzle the melted butter all over and stir well to combine. Pinch the mixture between your fingers and thumb. If it holds together on its own, the crust is ready. If it is sandy and falls apart, add 1 more tablespoon of melted butter and stir to combine.)

Place the mixture in the prepared baking pan. Use a flat-bottomed glass or measuring cup to firmly press the mixture into the bottom of the pan. Bake for 10 to 12 minutes, until golden brown around the edges and dry all over. Set aside to cool slightly.

Decrease the oven temperature to 325°F.

In a large mixing bowl with an electric mixer, beat the egg yolks for 1 minute until slightly thickened and lighter in color. Add the sweetened condensed milk and mix well to combine. Mix in the key lime juice and zest. Pour the filling over the crust.

Bake for 12 to 15 minutes until the filling is just set. Remove from the oven and let cool to room temperature. Refrigerate for 4 hours until well chilled.

Serve the bars with whipped cream, if you like. You can spread it over top of the bars in a thin layer, then slice and serve. Or, slice the bars and serve with a dollop of the whipped cream. Garnish with a slice of key lime or a sprinkle of lime zest.