For the batter: Beat egg whites and lemon juice until very stiff, then beat in sugar. Fold in almonds and cocoa.
Pour batter into a piping bag with large round spout. Pipe disks of batter, about 2 cm (approximately 3/4 inches) in diameter, onto a baking sheet lined with parchment paper. Bake in an oven preheated to 130°C (approximately 275°F) with oven door ajar for about 40 minutes.
For the filling: Chop chocolate and melt over a double boiler. Beat cream until stiff. Whisk pieces of butter into chocolate. Let chocolate cool, then fold into cream.
Coat the bottom of half the macaroons with filling and sandwich with remaining macaroons. Melt remaining chocolate and use to decorate macaroons. Let set, then serve.