Double Chocolate Shortbread

• 1 cup unsalted butter softened
• 1/2 cup icing sugar
• 1/4 teaspoon vanilla
• 1 1/4 cup all-purpose flour 
• 1/4 cocoa powder
• 1/3 cup cornstarch
• 1/4 teaspoon salt
• glacé cherries halved (optional)
• pecan halves (optional) 

In large bowl, beat butter until smooth. Beat in icing sugar until very fluffy and pale yellow, about 3 minutes. Beat in vanilla. In separate bowl, whisk together flour, cocoa powder, cornstarch and salt; stir into butter mixture in 3 additions to form soft dough. Stir in 1 cup dark chocolate chips into dough.

Drop by 1 tbsp, 2 inches (5 cm) apart, onto parchment paper–lined rimless baking sheets. Or roll into balls and use fork tines to flatten and make indentations on the top of each cookie. Or fill piping bag fitted with star tip and pipe dough into rosettes. Top with cherry or pecan half, if desired.

Bake, 1 sheet at a time, in 350°F (180°C) oven until light golden, about 12 minutes.
Let cool completely on pans. (Make-ahead: Store in airtight container for up to 24 hours.)