Melt butter and 2 ounces semisweet chocolate in a microwave. Meanwhile, put egg, espresso powder, and sugar into a bowl; whisk for about 1 minute or until light. Combine melted-chocolate mixture with the egg mixture, scraping bowl if necessary. Sift together flour, baking powder, and salt; combine with the chocolate mixture, and mix well. Finely chop 2 ounces semisweet chocolate, and add to bowl, mixing until well-incorporated. Let chill for 15 minutes. Form a 6-inch log, and freeze for 3 hours or overnight. Preheat oven to 325°. Slice into 1/2-inch-thick slices, and place on baking sheet. Bake for 12 minutes. Cool on wire rack, and serve.