To make the cookies: Beat together the sugar, butter, salt, baking powder, and espresso powder until fluffy. Beat in the egg and vanilla., Whisk together the cocoas and flour, then beat into the egg-butter mixture until well blended., Shape the dough into a 1" thick rectangle, wrap it in plastic, and refrigerate for at least 30 minutes., While the dough is chilling, make the chocolate topping. Heat 1 cup of the white chocolate in the top of a double boiler (or in a microwave in 30-second increments) until almost fully melted., Add an additional 1/2 cup chocolate, stirring until smooth and fully melted; this is a basic way of tempering the chocolate.*, Place the chocolate molds from your butter cookie set on a baking sheet, and spoon the melted chocolate into the molds. You'll need about a heaping teaspoon of chocolate per mold., Once all the molds are filled, gently tap the pan to help remove any bubbles in the chocolate. Allow the chocolate to set until hard., Once the chocolate is hard, gently remove each piece from its mold, then refill the molds with chocolate (re-tempering it if necessary). Repeat the process until you've used up all the chocolate., To bake the cookies: Preheat the oven to 350°F. Lightly grease two baking sheets, or line with parchment., Sprinkle both sides of the chilled dough with extra cocoa, to keep the dough from sticking. Roll it 1/8" thick., Cut out rectangles of dough using the cookie stamp from your butter cookie set; it helps to dust the stamp with extra cocoa as well. Place the cookies on the prepared baking sheets., Bake the cookies for 8 to 10 minutes, until the edges are firm and you start to smell chocolate., Remove the cookies from the oven, and allow them to cool completely., To assemble the cookies: Gently spread a thin layer of melted white chocolate on the back of each chocolate piece (to act as glue), then place one on top of each cookie., Allow the chocolate pieces to harden in place before serving., Yield: about 2 dozen cookies., *For more detailed instructions on tempering, see our blog, Flourish.