Cream butter and sugar until light and fluffy, 5-7 minutes. Beat in egg yolk and vanilla. In another bowl, whisk flour, baking powder and salt; gradually beat into creamed mixture. Divide dough in half. Add melted chocolate to 1 portion; mix well. Cover and refrigerate both portions until chilled., Divide each portion of dough into fourths; shape each into a 5-in. log. Flatten into triangular-shaped logs. Brush long sides with milk. Assemble 1 large roll by alternating 2 chocolate and 2 plain logs. Repeat. Wrap and refrigerate until firm, about 4 hours., Preheat oven to 375°. Unwrap each roll; cut into 1/4-in. slices. Place 2 in. apart on lightly greased baking sheets. Bake until set, 8-10 minutes. Remove from pans to wire racks.