1
: Line 9-inch (23 cm) square pan with parchment paper, overlapping both sides for easy removal from pan.
2
: 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.
3
: Bake 20 minutes. Remove pan from oven.
4
: Topping: Stir all ingredients together in a bowl. Spread over shortbread base. Bake until bubbling, about 20 minutes. Remove from oven and cool completely on wire rack, about 2-3 hours (nut mixture will harden over several hours). Cut into squares.