Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper.
Grind the nuts with confectioners' sugar until powdery (do not make into a paste).
Blend together the butter, lemon zest, salt, and nut mixture in a large bowl with a spatula or spoon until creamy. Add the flour and cocoa powder, stirring until incorporated. Do not overwork the dough.
Roll level 1/2 teaspoons of dough into tiny balls and arrange 1-inch apart on the prepared baking sheets. Bake 1 sheet at a time until lightly crisped, about 12 minutes. Slide the parchment paper with the baked cookies onto a rack to cool.
In a microwave-safe bowl, melt the chocolate in the microwave.
Spoon a mound, about 1/8 teaspoon, of melted chocolate onto the flat side of a cookie then top with the flat side of another cookie to make a "sandwich." Repeat.
Plain cookies can be baked and cooled 1 week ahead and kept at room temperature in an airtight container. Fill when ready to serve.