Heat oven to 350°. Line two baking sheets with parchment paper; set aside. Process almonds in bowl of food processor until finely ground, 1 minute. Add confectioners' sugar and process for 2 minutes. Scrape bottom and insides of bowl to loosen clumped almond mixture, then continue to process until very finely ground, 3 minutes more; set aside.
Beat egg whites in the bowl of a standing mixer fitted with a whisk on medium-low speed until frothy, about 2 minutes. Increase speed to medium-high and beat until stiff, glossy peaks form, about 3 minutes more. Transfer whites to a large bowl. Fold in almond mixture a third time. Transfer to a large pastry bag fitted with a 1⁄2" plain pastry tip. Pipe 1" rounds about 1⁄2" apart onto baking sheets. Set aside for 10–12 minutes; bake 15–18 minutes, until stiff. Leave to cool.
Combine granulated sugar and 3⁄4 cups water in a 2-qt. saucepan over medium high; heat until a deep-fry thermometer reaches 242°, about 20 minutes. Meanwhile, whisk egg yolks in a medium bowl with a hand mixer; slowly add sugar syrup, then butter and cocoa powder. Mound 1 tbsp. to the bottom of buttercream onto each meringue. Freeze for 15 minutes. Pour enough water into a 4-quart saucepan that it reaches a depth of 1". Bring to a boil; reduce heat to low. Place chocolate in a medium bowl; set bowl over saucepan. Cook, stirring, until melted and smooth, about 5 minutes. Remove from heat; set aside. Dip cookies, buttercream side only, in chocolate to fully coat. Place meringue-side down on a baking sheet and refrigerate until chocolate has hardened.