Protein-butter cream on Swiss meringue

Ingredients:

• 3/4 cup sugar
• 2 tsp freshly squeezed lemon juice
• A pinch of fine salt
• Whites of 4 large eggs
• 350 g butter, cut into 4 cm pieces, room temperature
• Flavorings (see below)

Preparation:

1. In a saucepan that fits a heat-resistant mixer bowl, add a few centimeters of water and bring to a boil.
2. Place the sugar, lemon juice, salt, and egg whites in a bowl and whisk by hand. Place the bowl over simmering water and whisk until the mixture is warm to the touch and the sugar is completely dissolved. Transfer the bowl to a stand mixer fitted with a whisk attachment and beat on medium-high speed until the mixture cools and holds stiff peaks, 10-15 minutes.
3. Add the butter a couple of pieces at a time, mixing them completely before adding the next one. Once all the butter has been added, continue beating on medium-high speed. The mixture will separate and appear curdled. Continue beating until the buttercream reaches a smooth consistency. Slowly, without stopping the mixer, beat in the desired flavorings. (If the buttercream is too soft, refrigerate it until it's firm but still pliable, then beat it before using.)
   
   

   Chocolate:

   
   Transfer the cream to a large bowl and stir in 170 g of melted and cooled dark chocolate in three additions.
   
   

   Citrus:

   
   Stir in the zest of half a lemon or orange.
   
   

   Alcohol:

   
   Stir in 2 tablespoons coffee liqueur and 1 tablespoon vodka.