1. In small bowl, whisk flour, cinnamon and salt. In large bowl, beat butter and sugar until fluffy, 2 minutes. Add yolks and vanilla; beat until smooth. Stir in flour. Divide dough in half; roll each into a 6-inch log. Wrap in plastic and gently flatten sides of log so each resembles a fat stick of butter and, when cut, the slices will be square. Refrigerate at least 2 hours or overnight.
2. Heat oven to 350 degrees F. Unwrap logs. Cut each into 16 slices (about 1/3 inch thick). Place on ungreased baking sheets.
3. Bake at 350 degrees F for 15 to 18 minutes, or until lightly golden. Transfer cookies to rack; let cool.
4. In medium-size glass bowl, microwave chocolate on high for 30 seconds, stirring until smooth. Spread each flat cookie bottom with 1 teaspoon melted chocolate. Dry, chocolate side up, on wire racks (30 minutes). Once dry, remelt remaining chocolate in bowl. Transfer to a plastic bag; snip off a small corner. Flip cookies over (chocolate side down) and drizzle stripes. Let dry.