STEP 1:
Whisk egg whites and sugar over a double boiler. When the sugar is fully dissolved, beat the mixture in a stand mixer on medium-high speed, until stiff peaks form.
STEP 3:
Add softened butter to the meringue, 2 Tbsp at a time & beat until combined. Add the espresso mixture & hazelnut extract & mix until fully combined.
STEP 1:
Place sugar & water in a saucepan & swirl until sugar is fully immersed in water. Place over medium heat until sugar dissolves & comes to a gentle simmer.
STEP 2:
In the bowl of a stand mixer with the whisk attachment, beat eggs on high speed, then lower to medium speed & slowly drizzle in the hot sugar syrup.