Using a food processor, pulse graham crackers into fine crumbs. Add butter and pulse again to mix together.
Equally split the mixture among six 4-inch tart pans with removable bottoms; press firmly on bottoms and up sides.
Freeze until firm for at least 30 minutes.
In a medium saucepan, combine the cream and sugar and cook on medium to high heat. Continue stirring for 5 minutes. Let it boil, then reduce heat and simmer 1 minute.
Remove the saucepan from heat then add the lemon zest and juice.
Allow to cool for 20 minutes, then equally distribute among crust-lined tart shells and refrigerate until set for at least 2 hours or up to 24 hours.
In a large mixing bowl, whisk together the egg whites, sugar and cream of tartar.
Put the bowl over a saucepan of simmering water to cook the mixture.
Whisking constantly, until sugar has dissolved and whites are very warm to the touch (not hot and overcooked). Continue the process for 5 minutes.
Remove from heat and using an electric mixer, beat on low speed, gradually increasing to high, until glossy, soft peaks form. Do it for about 5 minutes.
Beat in vanilla. Spoon over tarts, then use a torch to lightly toast.