To make the filling, puree the mangoes, sugar, lemon juice, cardamom, and salt in a blender or food processor.
Mix the cornstarch with water. Pour the puree into a sauce pan, fold the cornstarch-water mixture in, and cook on medium high for about 10-12 minutes until mixture thickens. Whisk the mixture continuously while cooking to ensure lumps don't form. If the mixture is still not thick enough to your liking, you can add a tablespoon of cornstarch (mixed in water) at a time until it thickens to your liking. The filling is not meant to be super thick. The texture should be slightly thinner than pudding.
Refrigerate until cool. Once cooled, fold in the greek yogurt.
Preheat oven to 350 °F .To make the almond crusts, mix the room temperature butter with the almond flour, brown sugar, vanilla extract, and salt until fully combined and a dough forms,
Press dough into mini tart pans. First press dough onto sides and then on the bottom. Poke the bottom with a fork. Refrigerate for about 10 minutes and then pop them into the oven for about 30 minutes until they have nicely browned and firmed up. Cool before assembling tarts.
To assemble tarts, pour the mango filling into the crusts. Allow to cool 30 minutes before serving, if you can wait that long!