1. Preheat oven to 350°F (180°C). Line 8-inch (20 cm) square baking pan with enough parchment paper to leave overhang.
2. Using electric mixer, beat butter and icing sugar for 2 to 3 minutes or until softened; stir in flour. Pour shortbread mixture into bottom of pan and press to flatten evenly. Bake for 18 to 20 minutes or until lightly golden browned and firm. Let cool completely.
3. Combine caramel layer ingredients and blend with an electric mixer. Pour over hot shortbread base and bake until the middle is set (no longer jiggles) about 25-35 minutes. Remove from oven and allow to cool completely.
4. Once caramel layer has cooled, in small saucepan over medium-high heat, bring cream to boil. Reduce heat to low; add chocolate, and butter, whisking gently until chocolate is melted. Remove from heat; let cool until slightly thickened.
5. Spread chocolate mixture evenly over top; chill for at least 4 hours. Using parchment paper overhang, remove from pan. Cut into 24 pieces. Can be made ahead, covered and refrigerated for up to 2 weeks.