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).
Cool completely on baking sheets.
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