Directions

• For the mango rum curd
• In a medium saucepan bring the mango puree and the lemon juice to a boil. In the meanwhile in a medium bowl whisk the egg yolks and sugar until well combined, then add the cornstarch.
• Pour the warm mango mixture onto the eggs, whisking vigorously, so that they don’t scramble. Pour everything back into the saucepan and bring to a slow boil, while continuing whisking, to avoid lumps. Cook for a couple of minutes. Remove from the heat, add the butter, salt, then the rum. Whisk until well combined. Place in a medium bowl, cover with plastic wrap at contact and refrigerate.
• For the salted caramel sauce
• In a small microwave safe bowl, warm up the heavy cream. Place the sugar in a saucepan, and heat over medium heat. Slowly melt the sugar until it reaches a dark amber color. 
• Remove the pan from the stove and slowly pour in the cream while whisking it slowly. Be careful because it will splatter. If the caramel seizes up on you – because the cream is too cold of you poured it too quickly – just return the mixture to a low heat and cook it until the sugar crystals are fully dissolved.
• Add butter and salt. Whisk the mixture until the butter is fully melted. Transfer the the sauce into a bowl and cool completely.
  
• For the caramel Swiss meringue buttercream
• Place the egg whites, sugar, and salt in the bowl of a stand mixer and whisk until well combined. 
• Set the bowl over a pan of simmering water (the base of the bowl should not touch the water) and whisk constantly until the mixture reaches the temperature of 150 degrees Fahrenheit on a sugar thermometer. 
• Remove the bowl from the pan and put it in place on the mixer fitted with the whisk attachment. Whisk on high speed to make a thick white meringue. Keep whisking until the meringue has cooled to room temperature, then start adding the butter by the tablespoon, until it’s all incorporated and the buttercream is smooth and silky.
• Add the caramel sauce in three batches, and keep whisking until light and fluffy. Set aside.
• For the coffee walnut cupcakes
• Preheat the oven to 350 degrees Fahrenheit. Line two muffin pans with baking cups – you should have 18 to 24 cupcakes. Set aside
• Put the walnut pieces and the sugar in a food processor or a coffee grinder, and grind to a fine powder. In a bowl of a stand mixer with a paddle attachment mix the walnut mixture, the butter, flour, baking powder, baking soda, salt, and eggs, until you obtain a smooth batter.
• In a small bowl, combine the buttermilk, espresso powder, vanilla, and glycerin. Add to the flor mixture and beat until well combined and smooth, scraping down the sides when necessary.
• Drop dollops of the batter in the baking cups, filling them 3/4 of the way. Bake in the oven for 20 to 25 minutes, until they spring back when touched. Coll on a cooling rack for 10 minutes, then remove from the pan to cool completely.
• To assemble
• With a cupcake coring tool, or with a knife, make a whole in the center of the cupcakes, making sure to leave at least 1/4 inch of cake at the bottom. Mix the mango curd until smooth, and drop in the center of the cupcakes. ,Close the hole with the top of the cake you removed early.
• Place the caramel Swiss buttercream in a piping bag with a star tip. Decorate the cupcake, and drizzle with leftover caramel sauce, if you’d like.