-
1
Place egg whites in a small bowl; let stand at room temperature for 30 minutes.
-
2
Preheat oven to 350u00b0. Place hazelnuts and confectioners' sugar in a food processor; pulse until nuts are finely ground.
-
3
Add salt to egg whites; beat on medium speed until soft peaks form. Gradually add superfine sugar, 1 tablespoon at a time, beating on high until stiff peaks form. Fold in hazelnut mixture.
-
4
With a reusable pastry bag, pipe 1-in.-diameter cookies 2 in. apart onto parchment paper-lined
-
5
. Bake until lightly browned and firm to the touch, 9-12 minutes. Transfer cookies on the parchment paper to wire racks; cool completely.
-
6
For the buttercream, in a heavy saucepan, combine sugar and water. Bring to a boil; cook over medium-high heat until sugar is dissolved. Remove from heat. In a small bowl, whisk a small amount of hot syrup into egg yolks; return all to pan, whisking constantly. Cook until thickened, 2-3 minutes, stirring constantly; remove from heat. Stir in espresso powder and vanilla; cool completely.
-
7
In a stand mixer with the whisk attachment, beat butter until creamy. Gradually beat in cooled syrup. Beat in confectioners' sugar until fluffy. Refrigerate until mixture firms to a spreading consistency, about 10 minutes.
-
8
Spread about 1-1/2 tsp buttercream onto the bottom of each of half of the cookies; top with remaining cookies. Store in airtight containers in the refrigerator.
