These cookies are always a part of cookie plates at our house! They also make a great hostess gift, wrapped with a ribbon!
Over a pot filled with 2 inches of simmering water, place a metal or glass bowl and add butter. Stir until melted halfway and add chopped bittersweet chocolate. Stir until just melted and remove from heat. Set aside.
Whip eggs with sugar, vanilla and espresso powder with electric beaters or in a mixer fitted with the whisk attachment, until pale and thick, about 5 minutes. In a separate bowl, combine flour, baking powder and salt.
Whisk melted chocolate into egg mixture until incorporated. Whisk in flour mixture and stir in white chocolate chunks. Cover batter with plastic and chill for at least 4 hours before baking.
Preheat oven to 325 F.
Spoon cookie dough by tablespoonfuls (or teaspoonfuls for dainty cookies) and roll gently to shape into a ball. Roll cookie in icing sugar and place on a parchment-lined or greased cookie sheet, leaving 2 inches between cookies.
Bake for 18-20 minutes. To test doneness, lift a cookie off the tray – if it comes off cleanly, then cookies are done.
Cool cookies before storing. Cookies will keep up to a week at room temperature in an airtight container. Batter also freezes well.