Preheat oven to 350 degrees; line cookie sheet with parchment paper.
Whisk eggs, sugar, and vanilla in a small bowl and set aside.
In another bowl, combine flour, baking powder, espresso powder, and salt; set aside.
Melt butter and chocolate in the top half of a double boiler*, stirring frequently. When melted, remove from heat and slowly stir in egg mixture.
Add flour mixture and hazelnuts to chocolate mixture and blend well.
Spoon rounded tablespoons of cookie dough on the prepared baking sheet, and bake for 11-13 minuts or until the cookie surfaces look dry.
Allow to cool slightly before serving.
*Note: If you do not have a double boiler, you can create one simply by placing a heavy, heat-safe glass bowl on top of a saucepan of boiling water. When water begins to boil, reduce to a simmer, and place ingredients in the bowl to melt slowly.