Heat oven to 350°F Beat sugar, butter, vanilla and egg in a large bowl with electric mixer on medium speed, or mix with spoon. Stir in flour, cocoa and salt. Shape dough into 1-inch balls. Place about 2 inches apart on ungreased cookie sheet. Press thumb or end of a wooden spoon into the center of each cookie. Do not press all the way to the cookie sheet.
Bake 7 minutes to 11 minutes or until edges are firm. Quickly remake indentations with the end of a wooden spoon if necessary. Immediately remove from cookie sheet to wire rack.
Cool completely, about 30 minutes. Meanwhile, make Espresso Filling.
Spoon a rounded 1/2 teaspoon Espresso Filling into indentation in each cookie.
Mix whipping cream and instant coffee in 1-quart saucepan. Heat over medium heat, stirring occasionally, until steaming and coffee is dissolved. Remove from heat; stir in chocolate chips until melted. Stir in liqueur, if desired. Cool about 10 minutes or until thickened.