Melt chocolate chips in a bowl set over simmering water until smooth. Cool slightly.
Beat sugar with butter in a large bowl until well combined. Beat in eggs, one at a time. Beat in milk, vanilla and espresso powder (if using). Beat in melted chocolate.
Whisk flour with cocoa, baking powder and salt in a separate bowl. Mix flour mixture into sugar mixture until a sticky dough forms. Chill for 15 minutes or just until firm enough to roll.
Meanwhile, preheat oven to 350°F (180°C). Line baking sheets with parchment paper.
Pour granulated and icing sugars in two shallow bowls.
Roll 1-tbsp portion of dough into a ball and roll immediately in granulated sugar to coat, place on prepared baking sheet. Repeat with remaining dough and granulated sugar, placing balls 2 inches apart.
Re-roll each ball in icing sugar and return to baking sheet. Bake, in batches as needed, for 12 to 14 minutes or until just set (don’t overbake).