Chocolate Mint Triangles

Heat oven to 350 °F. Have baking sheet(s) ready. Beat butter, shortening and both the light-brown and granulated sugar in a large bowl with mixer on medium speed until well mixed. Beat in yolks, vanilla and salt until blended. On low speed, gradually add flour; beat until just blended. Beat chocolate into dough until blended. Divide dough in half; pat each half into a rectangle. Place 1 rectangle between 2 long sheets of plastic wrap. Roll dough with a rolling pin into 13 x 9-in. rectangle. Place on baking sheet and refrigerate 20 minutes or until firm. Peel wrap off top. Flip dough over onto the baking sheet; peel off wrap. Trim dough to a 12 x 8-in. rectangle. Cut in 2-in. squares, then cut each square diagonally into 2 triangles, leaving dough on pan. Refrigerate 10 minutes or until firm. Bake 13 minutes or until top of dough looks dry. Cut through previously cut lines. Cool on sheet on a wire rack 2 minutes before removing to rack to cool completely. Repeat with remaining dough. Beat ingredients with mixer on medium speed until blended. Scrape into a qt-size ziptop bag; cut tip off 1 corner. Pipe filling on half the triangles; top with another triangle.