Preheat the oven to 350 degrees F and line cupcake pans with papers.
Place the flours, sugar, baking powder, baking soda, lime zest, and salt in a large mixing bowl, and mix on low speed to combine.
Add the butter, stirring on low speed until the mixture resembles damp sand (about a minute).
Stir in the eggs, one at a time, scraping the sides and bottom of the bowl with a silicone spatula after each addition.
Add the mango puree, lime juice, and tequila (if using), and mix on medium-high speed for about a minute and a half, to build the cake’s structure and aerate the batter.
Place about ¼ cup of batter in each well of the cupcake pans, and bake the cupcakes for 16-18 minutes, or until they spring back when lightly pressed and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool completely and top with buttercream.
To make the buttercream:
Place half the sugar in a small pot with the water, and cook over medium heat.
Place the egg whites, cream of tartar, and salt in a large mixing bowl and whip on medium-high speed with the whisk attachment.
When the whites can hold soft peaks, slowly add the remaining sugar (about a tablespoon at a time) while continuing to whip.
When all the sugar has been added, add the vanilla and continue to whip the meringue until stiff. Turn the mixer down to low speed while you wait for the sugar syrup to come to temperature.
When the sugar syrup has reached 235 degrees F, remove it from the heat, turn the mixer back up to medium-high, and slowly pour the hot syrup down the side of the mixing bowl in a thin stream. (Do not pour onto the spinning whisk; it could splash out of the bowl and burn you.)
Continue to whip until there is no hint of warmth when you place your hand on the side of the bowl.
When the meringue is completely cool, whip in the butter, a tablespoon at a time. (If the mixture seems runny, place the entire bowl and whisk in the refrigerator for 10 to 20 minutes, and re-whip. If the mixture seems curdled, just keep whipping- it will come together.)
Whip in the mango puree, lime juice, and tequila, if using.