Assemble the sandwiches the day before, so they can freeze thoroughly.
Melt together chocolates and butter in a medium bowl over simmering water, stirring with a wooden spoon. When smooth, add the salt and sugar, stirring well. Remove from heat. Beat in eggs, then fold in flour until mixed. Let cool, roll in plastic wrap, and refrigerate for at least 1 hour.
Heat oven to 350 degrees. Line a baking sheet with parchment paper.
When dough is firm, cut off a large piece. Dust a work surface with sugar and roll dough 1/4 inch thick, sprinkling more sugar as needed to prevent sticking. Cut into desired shapes and bake on prepared sheet for 11 minutes. Cool wafers on a wire rack.