Heat oven to 350°F.

Spray cookie sheet with nonstick cooking spray.

Beat butter until creamy.

Add brown sugar and 3 tablespoons granulated sugar; beat until blended.

Add egg, baking soda, corn syrup and vanilla; beat well.

Stir together flour and cocoa; beat into butter mixture.

If batter becomes too stiff, use wooden spoon to stir in remaining flour.

Cover; refrigerate about 30 minutes, if necessary, until batter is firm enough to shape.

Shape dough into 1-inch balls.

Cool completely.