Preheat oven to 350° F and line baking sheet with parchment paper or nonstick cooking spray.
In a large bowl, cream together butter and sugar. Beat in vanilla and egg. Stir in flour, cocoa, and salt until just combined.
Scoop dough by rounded teaspoonful and shape into ball. Place onto prepared cookie sheet and press thumb into center to make indentation.
Bake at 350° for 7-11 minutes or until edges are firm. Remove from oven and let cool on baking sheet for 2-3 minutes before transferring to wire rack to cool completely.
Espresso Filling
In a small saucepan, heat heavy cream and instant espresso powder over medium heat, stirring occasionally until coffee is dissolved and cream is steamy. Remove from heat and stir in coffee liqueur and chocolate chips until melted.
Let cool for about 10 minutes to thicken then spoon into indentation of cookies.