Preheat oven to 350°F.
To prepare Ganache:
Place chocolate chips and half and half in a microwave-safe bowl. Microwave on HIGH power (100%) for 30 seconds to 1 minute or until chocolate begins to melt. Stir together until smooth. Set aside.
To prepare Cookie Dough:
Combine flour, corn starch, powdered sugar and cocoa powder in medium bowl; set aside.
Beat butter in a large bowl with mixer at medium speed until smooth. Add flour mixture and vanilla to butter and blend. Add 1/2 cup ganache (reserving remaining ganache for later use) to butter mixture and beat until well blended. (If necessary, refrigerate dough for 30 minutes to 1 hour or until easy to handle.)
Roll dough into 1-inch balls and place on ungreased baking sheets. Bake 10 to 12 minutes. Let cool on baking sheet for 2 to 3 minutes, then transfer to wire rack until cooled to room temperature.
Drizzle remaining ganache over cookies (see recipe tip below). Store cookies in air tight container.
Recipe Tip: Transfer ganache to a sealable sandwich-size plastic freezer bag using a rubber spatula. Seal bag and snip a small tip off the lower corner of bag. Squeeze ganache through opening to pipe onto cookies.