Line a 10-by-15-inch rimmed baking sheet with parchment paper. In a double boiler, heat the chopped chocolate over simmering water, stirring frequently, until almost melted and an instant-read thermometer registers 105 degrees , about 20 minutes. Remove the pan from the heat and stir until the thermometer registers 110 degrees , 1 to 2 minutes more. Remove the insert from the pan, reserving the pot of water. Let the chocolate cool, stirring occasionally, until the thermometer registers 87 degrees , 25 to 45 minutes.
Add the grated chocolate to the melted chocolate, return the insert to the pot and stir constantly over medium-low heat until the temperature reaches 90 degrees and most of the grated chocolate has melted. Remove the insert and stir in the almonds. Dry the outside of the insert, then pour the chocolate onto the prepared baking sheet; spread into an even layer, leaving a 1-inch border. Refrigerate for 20 minutes, then let stand at room temperature until hardened, about 45 minutes. Using a sharp knife, slice the chocolate block into uneven slabs of bark.