Line two or more baking sheets with parchment paper.
In a medium mixing bowl combine the sugar, butter, salt, baking powder and espresso powder. Beat in the egg and vanilla. Then add the flour and cocoa. Cover the dough and refrigerate for 30 minutes.
Preheat oven to 350 degrees.
Roll the dough out on a large piece of parchment paper to about 1/8 inch thick, using cocoa instead of flour to dust the rolling pin and surface. Cut into 2 1/2″ round cookies.
Bake for about 10 minutes taking care to check them near the 8 minute mark. If you smell chocolate before the 10 minutes is up, take them out to cool. Cool cookies completely on a wire rack. Store in an airtight container.
To make the stack cakes whisk together the cream, sugar, cocoa and espresso. Beat until soft peaks form. Layer the cake stacks starting with one cookie on the bottom, one heaping teaspoon of the filling, top with a second cookie pressing slightly to compress the whipped cream to about 1/4″ thick. Repeat with 4 more cookies. Refrigerate 24 hours before serving. Add a dollop of whipped cream and garnish with chocolate shavings just before serving.