Preheat oven to 140°C (approximately 285°F.)
For the cookies: Whip egg whites with lemon juice until very stiff peaks form. Gradually sprinkle in sugar and continue beating until mixture is cut-resistant and glossy. Carefully fold in almonds and cocoa powder. Transfer mixture into piping bag with large, round nozzle.
Pipe small circles of cookie batter about 2 cm (approximately 3/4 inch) in diameter onto baking sheet lined with baking paper. Bake cookies in preheated oven for about 25 minutes, leaving oven door open slightly.
For the filling: Chop dark chocolate into small pieces and gently melt over hot, not boiling water.
Whisk cream with butter cut in small pieces. Add melted chocolate and let filling cool slightly.
Cover half the cookies with filling and top with remaining cookies. Serve macarons dusted with powdered sugar.