Preheat oven to 350°F (177°C). Lightly grease four 6-ounce ramekins with butter or nonstick spray. See recipe note about other ramekin sizes. Place in a baking pan or small casserole dish. Four 6-ounce ramekins fit nicely into a 9-inch square bake pan. The pies will bake in a water bath inside the pan.

In a medium bowl using a handheld or stand mixer fitted with a whisk attachment, beat the egg yolks on medium-high high speed until pale and slightly thickened, about 3-4 minutes. Add the lime juice, sweetened condensed milk, and lime zest. Beat on medium speed until combined. You’ll have about 2 cups of filling. Pour evenly into prepared ramekins.

Without splashing into the ramekins, slowly pour hot water into the casserole dish around the ramekins, about halfway up the sides of the ramekins. If it’s easier, you can transfer the casserole dish to the oven first, then carefully pour in the hot water.

Carefully place the casserole dish/baking pan in the oven. Bake until the top of each is mostly set, about 17-20 minutes. When lightly tapped, the very center should only slightly jiggle. Though not necessary, you can also check the internal temperature. These are safe to eat when an instant read thermometer reads 160°F (71°C).

Using tongs or an oven mitt, remove the pies from the water bath and place on a wire rack set over a baking sheet or kitchen towel to catch any dripping water. Cool pies for about 1 hour at room temperature, then chill in the refrigerator for 1 hour before assembling and serving. You can enjoy at room temperature, but they taste better if you chill in the refrigerator.

As the pies cool, crush up the graham crackers with your hands or place in a zipped-top bag and crush with a rolling pin. Place butter in a skillet on the stove over low-medium heat. Add graham cracker crumbs and brown sugar. Stirring constantly, toast/cook for 4 minutes. Remove from heat and allow to cool.

Sprinkle a couple spoonfuls of crust topping on each cooled pie. Before serving, top with whipped cream, meringue, or any desired topping. I used Wilton 8B piping tip to pipe whipped cream in these photos.

Cover leftover pies tightly and refrigerate for up to 3 days.