Melt chocolate and butter together in a double boiler. Whisk until smooth. 

  Mix together cocoa powder, flour, baking powder, and salt.

 In an electric mixer, whisk together eggs and sugar for 2 minutes. Reduce to low speed and stir in the melted chocolate, coffee and vanilla. 

 Stir in dry ingredients. Refrigerate the dough until firm, about an hour.

 Preheat oven to 350° F.

 Scoop or form the cookies into balls, each about 1 tablespoon.

 Put confectioners (or powdered) sugar in a bowl and roll each of the balls in the sugar. Place on parchment lined baking sheets several inches apart.

 Bake for 13-15 minutes until crackled on top. Remove from the oven and gently press a walnut half in the middle of each warm cookie. 

Once cool, store in an airtight container.

Source: Chef Emily Luchetti and California Walnut Board