Line 3 cookie sheets with parchment paper. Beat the butter and superfine sugar together in a bowl until pale and fluffy. Beat in the egg yolk, then the amonds and flour. Continue beating until well mixed. Shape the dough into a ball, then wrap in plastic wrap and let chill in the refrigerator for 1 1/2 - 2 hours.
Preheat the oven to 325 degrees. Unwrap the dough and break off walnut size pieces, then roll them into balls between the palms of your hands. Place the dough balls on the prepared cookie sheets, spaced well apart to allow for spreading. Bake in a preheated oven for 20-25 minutes, or until golden brown. Remove from the oven and carefully transfer the cookies, still on the parchment paper, to cooling racks to cool.
Place the chocolate in a small heatproof bowl set over a pan of gently simmering water and stir constantly until melted. Remove the bowl from the heat. Remove the cookies from the parchment paper, and spread the melted chocolate over the bases. Sandwich them together in pairs and return them to the cooling racks to cool and set. Dust with a mixture of sifted confectioners' sugar and cocoa and serve.