1
: Preheat oven to 350ºF (180ºC).
2
: Line a 9-inch (23 cm) square pan with parchment paper, overlapping both sides for easy removal from pan.
3
: Shortbread Base: Sift all ingredients, except butter, together in a medium bowl. In a separate bowl, use electric hand-held or stand mixer to beat butter until creamy. Using a low speed, add in dry ingredients and mix until thoroughly combined. Gather into a ball. Press firmly and evenly into bottom of pan. Using a fork, poke the dough randomly about a dozen times.
4
: Bake 20 minutes. Remove pan from oven, and reduce heat to 325ºF (160ºC).
5
: Combine all filling ingredients together in stand mixer or blender. Pour over hot shortbread base and bake until middle is set, about 25 – 30 minutes. Remove from oven and cool completely on wire rack. Refrigerate until cold, about 2 hours.
6
: To serve, cut into squares and garnish with a light dusting of icing sugar.