First prepare the lemon curd, by zesting both lemons. Squeeze the juice from them, adding in an additional lemon if necessary, until you reach ¼ cup.
In the bottom pot of a double boiler add ½ cup of water over medium heat and bring to a simmer. In the top pot, add the butter and sugar and stir until melted.
Reduce heat to medium low and add in the lemon juice and zest. In a small bowl, lightly beat the egg.
Removing the top pot from the heat, pour the egg in while whisking constantly. Return to the heat and increase back to medium. Stir every few minutes until the mixture has thickened enough to coat the back of a spoon.
Place a fine mesh sieve over a bowl and strain the mixture through, discarding any solids. Cover the bowl with plastic wrap, pressing it against the top of the curd so a skin doesn’t form and refrigerate for at least an hour.
Preheat the oven to 350 degrees.
Prepare the meringue by adding the egg whites, cream of tartar and vanilla extract to the bowl of a stand mixer and beating on medium high speed until soft peaks form. Then pour in the sugar, a tablespoon at a time until fully combined. Increase the speed to high and beat until the mixture is glossy and holds stiff peaks. Transfer the meringue to a piping bag fitted with a decorative tip.
Place the phyllo cups on a baking sheet lined with parchment paper. Add a heaping tablespoon of the lemon curd into each phyllo cup and top with the meringue.
Bake for 7-10 minutes or until the meringues are lightly browned.
Let cool completely and then refrigerate until ready to serve.