Position racks in the upper and lower thirds of the oven; preheat to 250 degrees . Line 2 cookie sheets with parchment. Using a 3 1/2 inch round cookie cutter or small glass as a guide, draw 6 circles 1 inch apart on each parchment sheet.
Using a electric mixer with a whisk attachment, beat egg whites, cream of tartar and salt at medium speed until frothy, about 1 minute. With the machine running, gradually sprinkle in the sugar 1 tbsp at a time. When all the sugar has been added, beat at high speed until glossy and stiff, about 3 minutes. Fold in the lemon extract.
Transfer the meringue to a pastry bag fitted wtih a 1/2 inch closed star tip. Starting at the center of each traced circle and working outward, pipe 6 rounds onto each sheet.
Transfer the cookie sheets to the oven. Immediately lower the temperature to 200 degrees ; bake the meringues for 45 minutes. Quickly switch and rotate both pans. Bake for 30 minutes longer (don't open the oven). Turn off the oven (do not open it) and let the meringues cool in the closed oven for 6 hours. (They can be made 2 days ahead; store in airtight container.)
Let the ice cream soften until spreadable. Fold the graham crackers into the ice cream.
Place 6 of the meringues directly under a preheated broiler (or use kitchen torch) until the ridges are golden, about 2 minutes.
Spread a generous 1/3 cup of the graham cracker ice cream onto the flat side of each of the browned meringues. Spread 2 tbsp lemon curd onto the flat side of the remaining meringues, then sandwich the meringues together. Serve immediately or freeze for up to 2 hours.