Preheat your oven to 350°F (177°C). Crush 2 sheets of graham crackers into fine crumbs to get about ¼ cup. This can be done either by placing them in a ziplock bag and rolling them with a rolling pin or heavy can, or by pulsing them in a food processor.
Combine Crust Ingredients: In a small bowl, mix the graham cracker crumbs with sugar and melted butter.
Form the Crust: Transfer the mixture to a 10-ounce ramekin. Press the crumbs down gently, using your fingers or the back of a spoon, to form a crust at the bottom.
Bake the Crust: Place the crust in the oven and bake for 15 minutes. After baking, set it aside to cool.
Make the Filling
Increase the oven temperature to 400°F (200°C). In a separate bowl, whisk the egg yolk and set aside.
Cook the Filling Base: In a 1-quart saucepan, combine sugar and cornstarch. Gradually add water and cook over medium heat, stirring constantly, until the mixture thickens and starts to boil. Reduce the heat to low once thickened.
Temper the Egg Yolk: To avoid cooking the egg yolk, temper it by gradually stirring 1 to 2 spoonfuls of the hot mixture into it. Then, slowly whisk this tempered egg yolk back into the saucepan. Cook over medium heat until the mixture thickens and bubbles, stirring continuously. Remove from heat and stir in butter, lemon juice, and zest.Note on Tempering: Tempering gradually raises the temperature of the egg to prevent it from scrambling. Start by adding a small amount of the hot mixture to the egg yolk, whisking constantly, then slowly add more.
Add Filling to Crust: Pour the lemon filling over the prepared crust.
Make the Meringue
Beat the egg white and cream of tartar using an electric mixer on medium-high speed until foamy, about 2 minutes. Gradually add sugar, ½ tablespoon at a time, and continue beating until the mixture is stiff and glossy. Be careful not to overbeat, as this can make the meringue grainy.
Top Pie with Meringue: Spoon the meringue over the pie filling. Spread it evenly, making sure to seal the edges to prevent shrinking. Use the back of a spoon to create decorative peaks in the meringue.Note on Shrinking: Shrinking occurs when the meringue topping pulls away from the crust or the filling. To prevent this, ensure the meringue covers the edges of the pie completely.
Bake the Pie: Bake for 8-10 minutes, or until the meringue is golden brown.
Cool the Pie: Transfer the pie to a wire rack and let it cool to room temperature. This step is crucial as cutting the pie while warm can cause the filling to be runny.
Refrigerate: Once cooled, cover the pie and refrigerate until ready to serve. Store any leftovers in the refrigerator.
Notes
For a flawless meringue that is light, fluffy, sweet, and holds its shape, follow these essential tips:
Clean Equipment: Ensure your mixing bowl and whisk or beaters are completely clean and dry. Any residue can interfere with the meringue's ability to form peaks.
Room-Temperature Egg Whites: While cold eggs separate more easily, let the egg whites reach room temperature before whipping. Room-temperature whites whip up faster and more voluminously.
Avoid Egg Yolk Contamination: Be vigilant to prevent any yolk from mixing with the whites. Even a small amount of yolk can hinder the whites from whipping properly.
Gradual Sugar Addition: Add sugar gradually while whipping the egg whites. This helps it dissolve thoroughly, ensuring a smooth and glossy meringue.