Espresso Icebox Coins

Stir together espresso and boiling water. Beat butter and sugar with a mixer until pale and fluffy, about 3 minutes. Beat in vanilla and espresso. Add flour and salt, and beat until just combined. Beat in chocolate.

Divide dough in half, and shape into two 10-inch-long logs, about 1 1/4 inches in diameter. Wrap each in parchment, and freeze for 2 hours.

Preheat oven to 325 degrees. Let dough stand at room temperature for 5 minutes. Slice into 1/4-inch-thick rounds, and space 1 inch apart on parchment-lined baking sheets. Bake until centers are set and edges are golden, about 12 minutes. Let cool on wire racks. Before serving, dust with cocoa powder.