1) Beat together the eggs, sugar, vanilla, espresso powder, and salt till smooth.
2) Add the cocoa and baking powder, again beating till smooth.
3) Add the flour, mixing till well combined. Add the melted butter, again mixing till well combined.
4) Bake pizzelle according to your pizzelle iron instructions. A tablespoon cookie scoop works well for scooping batter onto the iron; a level scoopful of batter is the right size for most standard pizzelle makers.
5) To make two-tone pizzelle: Prepare plain pizzelle batter, flavoring it with 1/2 teaspoon hazelnut flavor, if desired. Using a teaspoon scoop (which holds 2 level measuring teaspoons), drop a chestnut-sized ball of vanilla batter just below center in the cooking area of the pizzelle iron. Drop a slightly smaller ball of chocolate batter just above the center line, nestling it right next to the vanilla batter. Close the iron, and bake according to the manufacturer's directions.