Preheat the oven to 200°F. Line a small sheet pan with parchment paper or a silicone mat.
In a medium-size bowl, using an electric mixer on medium speed, beat the egg white and cream of tartar until soft peaks form. Then, slowly stream in the sugar while beating. Beat until the mixture has stiff peaks.
Using two spoons or a piping bag, make four equal-size disks about 4 inches in diameter of the meringue mixture on the prepared sheet pan.
Bake for 40 minutes, and then turn off the oven and let the meringues cool inside the oven for 1 hour. Do not open the oven door.
To make the lemon curd:
In a medium-size bowl, using an electric mixer on high speed, beat together the butter and sugar until well combined. Add the egg and egg yolks one at a time while beating constantly. Finally, stream in the lemon juice and salt. The mixture may look a bit curdled – it’s fine.
Pour the lemon curd into a small saucepan and bring to a simmer over medium-low heat to thicken. Stir constantly to avoid burning. Once simmering, remove from the heat and let cool.
To make the whipped cream:
In a separate medium-size bowl, using an electric mixer on high speed, whip the cream with the powdered sugar until soft peaks form. Set aside.
Dollop the lemon curd on top of each meringue, add a spoonful of whipped cream, and sprinkle graham cracker crumbs on top before serving.