Preheat the oven to 350F.
Cream butter and sugar together.
Add flour and baking powder.
Pat into 8x8" pan lined with parchment paper.
Bake for 10 minutes. Set aside to cool.
Mix all caramel ingredients together in a medium saucepan.
Bring to a boil over medium-high heat. Once the mixture begins to bubble, lower heat immediately to low to allow caramel to continue to simmer. Simmer 5 minutes, stirring frequently.
Remove from heat and stir vigorously for 3-5 minutes, allowing the caramel to thicken.
Pour the caramel over the shortbread base. Refrigerate for about an hour to allow the caramel to cool and set.
Melt the chocolate chips in the microwave in 45 second intervals until the chocolate is melted and smooth.
Poor the chocolate over the caramel layer, distributing the chocolate evenly.
Sprinkle the top layer with fleur de sel while the chocolate is still warm, if desired.