for the filling, stir the butter and sugar in a pan over medium heat until the sugar has dissolved but not caramelized. Add the chocolate and stir until smooth.
Mix cocoa, almonds, and cornstarch in medium bowl. Stir in the chocolate mixture. Finally, stir in the eggs. Beat the mixture briefly until well mixed. Set aside but do not refrigerate.
Place the pastry on floured work surface. Dust the top with flour and roll out into a rectangle about 1/8 inch thick. Using a plain 3 inch cutter, cut out 16 rounds of pastry.
Spoon 2 tablespoons of the filling into the center of eight of the pastry rounds. Brush the egg around the edges of the pastry and place the remaining pastry rounds over the filling, pressing down gently to seal.
Using the back of a pastry cutter two sizes smaller, carefully center it over the top and press down firmly. This will leave a mark and prevents the Pithivier from opening during the baking.
Preheat the oven to 400°. Line 2 baking sheets with parchment paper. Place the pithiviers on the baking sheet about 1 inch apart. Chill in refrigerator for 15 minutes. Brush the tops with egg and use the back of a knife to mark curved lines from the center to the edge of the pastry all the way around each Pithivier. Bake until Golden Brown, 25 to 30 minutes. Transfer to a wire rack and let cool. Serves eight