Pre-heat the oven 200C / 400F / gas 6. Line 2 large baking sheets with parchment paper.
Pulse the almonds and 500 g icing sugar in a food processor until finely ground. Add cocoa and remaining icing sugar pulse until well blended.
Beat egg whites and salt in a large bowl with mixer on medium-high speed until whites form soft peaks when beaters are lifted. Add granulated sugar and beat just until stiff peaks form when beaters are lifted. With a whisk or rubber spatula, gently fold in almond mixture. Transfer to a piping bag and pipe 1-in.-diam mounds about 2 in. apart on prepared baking sheets.
Bake, 1 sheet at a time, 6 to 8 minutes, until tops look dry but macaroons are still slightly soft to the touch.
Transfer, still on parchment or foil, to barely dampened kitchen towels. Let cool 5 minutes. Carefully peel paper off macaroons and transfer to wire racks to cool completely.
Sandwich with chocolate spread and gently press together