1. Preheat oven to 350 degrees F. Line an 8- or 9-inch square pan or a 7-by-11-inch pan with parchment paper, letting ends of paper extend over edges of pan, or spray pan with vegetable cooking spray.
2. Prepare shortbread base. In a large bowl, use an electric mixer on medium speed to cream butter and sugar until smooth and lightened in color, about 2 minutes. Scrape down sides and bottom of bowl and beaters as needed with a rubber spatula. Mix in vanilla. Sift together flour, cornstarch, and baking powder and add to bowl. Mix on low speed until dough forms large clumps and holds together, about 1 minute.
3. If dough is too heavy to clump together with the mixer, use your hands to push it together.
4. Using your fingers, press dough gently and evenly into prepared pan. Bake until top is golden and edges brown, about 25 to 30 minutes.
5. While shortbread bakes, prepare caramel filling. Put butter, sugar, syrup, and condensed milk in a large saucepan and cook over medium heat until mixture comes to a boil, stirring occasionally, about 10 to 15 minutes. Continue boiling, stirring gently and constantly, 4 more minutes. Mixture should darken to a light caramel color. Be careful, as mixture may splatter. Remove pan from heat and pour filling over shortbread.
6. Let caramel filling cool for 5 minutes, then sprinkle chocolate chips over filling. Let chips soften until they melt, about 5 minutes. Use a knife or spatula to spread melted chocolate over filling.
7. Let chocolate set until firm, about 2 to 3 hours. When set, cut shortbread into 16 squares. Remove from pan and separate the pieces. Store in a tightly sealed container at room temperature for up to 3 days.
Source: Hannaford fresh Magazine, May - June 2009