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1
Preheat the oven to 320u00b0 F (160u00b0 C). Sift sugar, flour, and cornstarch together in a large bowl.
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2
In a separate bowl, cream together the butter, vanilla, and lemon zest. When creamy and soft, combine with the flour mixture and begin folding together with a spatula or by hand. Continue combining the mixture until you have a perfectly smooth, soft ball of dough. It will take a few minutes by hand.
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3
Roll into walnut-sized balls (try to get the same size each time; about 2 level teaspoons-worth is ideal) and place on a baking sheet lined with parchment paper about 1 1/2 to 2 inches apart.
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4
With the tines of a fork (easier if you dip the fork into flour each time), gently flatten each ball until the cookie is about 1/2-inch thick (it will spread a little more when baking and you do want these fairly thick rather than thin).
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5
Bake for 15 minutes, or until the cookies are still very pale but feel dry to the touch. They will still be quite delicate and soft, so let them stand on the tray for 5 minutes before carefully transferring to a wire rack to cool completely. They will harden when cooled.
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6
Make the mango buttercream by whipping the butter, sugar, and mango puree together until smooth and creamy. Place a neat teaspoon of buttercream in the center of the bottom of a cookie and carefully sandwich another cookie on top, pressing gently until the buttercream spreads nearly to the edges. Let them set in the fridge in an airtight container for 30 minutes before serving. They will keep a few days stored like this, but make sure to bring them back to room temperature before serving. Plain cookies without the buttercream will keep 1 week in an airtight container.
