Preheat oven to 350°. Stir together first 4 ingredients in a bowl. Stir in candy bar chunks.
Beat butter at medium speed with an electric mixer until creamy. Gradually add sugar, beating until blended and stopping to scrape down sides as needed. Add eggs, 1 at a time, beating until blended after each addition. Add flour mixture (1 cup at a time) alternately with liqueur (1 Tbsp. at a time). Beat until blended after each addition.
Divide dough in half. Roll each dough half into a 16-inch-long log, and place on a greased baking sheet. Flatten each log to 3/4-inch thickness. (Dough will be soft and sticky.)
Bake at 350° for 20 to 25 minutes or just until golden. Remove baked logs, keeping oven on.
Cut each log crosswise into 3 equal sections. Transfer each section to a cutting board, and cut into 1/2-inch-thick slices, wiping knife clean after every 2 cuts. Transfer biscotti to baking sheet, cut sides up.
Turn oven off. Let biscotti stand in oven, with door closed, 1 hour.
Note: For testing purposes only, we used Hershey's Skor Bars for milk chocolate-covered buttery toffee candy bars, Hershey's Symphony Bars for milk chocolate candy bars, and Frangelico for hazelnut liqueur.