1. In large bowl, with electric mixer on medium, beat butter and sugar until mixture is fluffy. Beat in cocoa, espresso powder, vanilla, and salt until well-combined. Beat in flour until just combined, then pecans. Chill dough, covered, at least 2 hours, or overnight.
2. Heat oven to 375 degrees F. Roll dough into 1-inch balls, and arrange 1 inch apart on baking sheets. Bake cookies 12 to 15 minutes, or until just firm; let cool 5 minutes on sheets. Toss warm cookies in powdered sugar, coating well.