Heat oven to 350°F. In large bowl, beat sugar, butter, vanilla and egg with electric mixer on medium speed, or mix with spoon, until well blended. Stir in flour, cocoa and salt until dough forms. Shape dough by rounded teaspoonfuls into 1 inch balls. On ungreased cookie sheets, place balls about 2 inches apart. Press thumb or end of wooden spoon into centre of each cookie, but do not press all the way to cookie sheet. Bake 7-11 minutes or until edges are firm. If necessary, quickly remake indentations with end of wooden spoon. Immediately remove cookie sheets to cooling racks. Cool completely, about 30 minutes. Meanwhile, in 1 quart saucepan, mix whipping cream and instant coffee. Heat over medium heat, stirring occasionally, until steaming and coffee is dissolved. Remove from heat; stir in liqueur, if using. Cool about 10 minutes or until thickened. Spoon rounded 1/2 teaspoon espresso filling into indentation in each cookie. Top with crushed candies.