Preheat oven to 325 degrees F (165 degrees C). Lightly butter (or spray with Pam) a 15 x 10 inch (38 x 25 cm) baking pan. 

In a small bowl, sift the confectioners sugar with the cocoa powder. Set aside. 

In another bowl, sift the flour, cornstarch and salt together.  Set aside.

In the bowl of your electric mixer (or with a hand mixer), cream the butter until smooth (about one minute). Add the sugar and cocoa mixture and beat until fully incorporated (about 2 minutes). Add the vanilla and espresso powder and beat until incorporated. Then add the flour mixture to the butter mixture and beat until the dough just comes together.

Spread the dough over the bottom of the prepared pan and gently press the dough, with your fingertips or the back of a spoon, to form an even layer. Then, with a sharp knife, score the top of the shortbread into approximately 3 inch x 1 inch (7 x 2.54 cm) fingers. Bake for 12 to 15 minutes, or until the top is dry and a toothpick inserted in the center comes out clean. Remove from the oven and place on a wire rack to cool. While the shortbread is still warm, re-cut the shortbread into fingers. Cool completely before removing from pan. 

If you want to garnish the shortbread fingers with white chocolate; in a small heatproof bowl over a saucepan of simmering water, melt the white chocolate with the vegetable shortening. Then place the melted chocolate into a paper cone or pastry bag fitted with a 1/16 inch tip. Carefully pipe a thin line of chocolate across the center of each cookie and then pipe another line across the first one to make them look like presents. Don't worry about making your lines exact.

Makes about 50 - 3 x 1 inch (7 x 2.5 cm) shortbread fingers

This recipe comes from a great little baking book called "Cindy's Itty Bitty Baking Book.