Cream butter and sugar until light and fluffy. 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 one portion; mix well. Refrigerate 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 one large roll by alternating two chocolate and two plain logs. Repeat. Wrap in plastic. 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.