Striped Slice-and-Bake Shortbread

Whisk together flour and salt in a small bowl. Beat butter, powdered sugar, and granulated sugar with a stand mixer fitted with a paddle attachment on medium speed until light and fluffy, about 3 minutes. Add vanilla extract, and beat until blended, about 10 seconds. Reduce speed to low, and gradually add flour mixture, one-third at a time, beating just until blended after each addition, about 1 minute.

Turn dough out onto a lightly floured surface, and knead a few times until dough comes together. Halve dough, and set 1 portion aside for plain dough. Return remaining dough to mixer, and add cocoa, beating on medium speed until blended, about 2 minutes. Divide chocolate dough and plain dough each into thirds, and roll each piece into a 15-inch rope. Stack ropes alternately on top of each other, flattening each slightly into a strip to cover previous layer. Roll dough into an even log by gently pushing each end toward center of log and rolling as needed. Wrap in plastic wrap, and refrigerate 2 hours. 

Preheat oven to 350°F. Line 2 baking sheets with parchment paper. Trim log so ends are flat, and slice half of log crosswise into ¼-inch-thick slices. Arrange cookies on prepared baking sheets, spacing them 2 inches apart. Place remaining unsliced log of dough and 1 baking sheet of cookies in refrigerator.

Bake remaining baking sheet of cookies in preheated oven until set and lightly golden on bottoms, about 14 minutes, rotating baking sheet from front to back halfway through bake time. Let cool on baking sheet 5 minutes; transfer cookies to a wire rack to cool completely, about 30 minutes. Meanwhile, bake remaining baking sheet of cookies as directed. Repeat slicing and baking procedure with remaining dough log in refrigerator, lining baking sheets with a new sheet of parchment paper for each batch. Reserve baking sheets, and line each with a new sheet of parchment paper. 

Place sprinkles in small bowls. Melt semisweet chocolate chips according to package directions, and stir in oil. To decorate, dip edge of each cookie in chocolate mixture, and top immediately with sprinkles. Place on prepared baking sheets, and let stand until chocolate sets, about 30 minutes. Store in an airtight container until ready to serve.