- Position a rack in the middle of an oven and preheat to 400F. Line 2 large baking sheets with parchment paper.
- To make the macaroons, in a food processor, combine the almonds with 1 cup of the confectioners' sugar and pulse until finely ground. Add the cocoa powder and the remaining 3/4 cup confectioners' sugar and pulse until well blended.
- In the bowl of an electric mixer, beat together the egg whites and salt on medium-high speed just until the whites form soft peaks when the beaters are lifted. Add the granulated sugar and beat just until the whites form stiff peaks when the beaters are lifted. Using a whisk or a rubber spatula, gently fold in the almond mixture.
- Transfer the batter to a pastry bag fitted with a 1/2-inch plain tip. Pipe out 1-inch-diameter mounds about 2 inches apart on the prepared baking sheets. Bake, one sheet at a time, until the tops are cracked and appear dry but the macaroons are still slightly soft to the touch, 6 to 8 minutes.
- Transfer the cookies, still on the parchment paper, to barely dampened kitchen towels and let cool for 5 minutes. Carefully peel the paper off the macaroons, transfer to wire racks and let cool completely. (The macaroons can be made 1 day in advance and stored in layers separated by waxed paper in an airtight container.)
- To make the filling, in a saucepan over high heat, bring the cream just to a boil. Remove the pan from the heat and whisk in the cocoa powder. Add the chocolate and butter and whisk until smooth. Let cool to room temperature, then cover and refrigerate until the filling is firm enough to hold its shape when spread, at least 30 minutes.
- If desired, transfer the filling to a pastry bag fitted with a 1/2-inch plain tip. Pipe the filling, or spread it with a table knife, generously on the flat side of half of the macaroons. Top with the remaining macaroons, flat side down, pressing together gently to form sandwiches. (The cookies can be stored in layers separated by waxed paper in an airtight container in the refrigerator for up to 2 days.) Makes about 4 dozen cookies.