Preheat over to 350°. Line two baking sheets with parchment paper.
Melt chocolate and butter in a double boiler, stirring occasionally, until a smooth mixture forms. Alternatively, melt in a heat-proof bowl in the microwave in short 15-20 second bursts until melted, stirring after each burst. While chocolate and butter are melting, combine eggs, vanilla, and sugar in a large bowl and set aside.
Sift together flour, baking powder, and espresso. Whisk chocolate and butter into the egg mixture until well combined. Gradually stir in flour, baking powder, and espresso until a smooth batter forms. Chill batter in the fridge for an hour, or until firm.
Drop tablespoons of batter onto prepared baking sheets, spacing at least 2 inches apart. Bake for 8–10 minutes, until crackled and firm on the outside. Don't overbake!