Preheat oven to 350 degrees. Line a glass 8×8 pan with parchment paper.
Mix all shortbread ingredients together thoroughly. Spread dough evenly across bottom, pushing up the edges slightly higher to form a crust for the lemon filling. Bake for 15-18 minutes, or until sides are gold brown
While shortbread is baking, make filling. Whisk together eggs and one yolk, then add in honey, lemon juice, and left zest. Whisk together gently. In a small bowl, mix arrowroot flour with 1 tbsp water until no clumps remain, then add to egg mixture.
When shortbread comes out of oven, reduce temperature to 325 degrees and immediately pour lemon filling over top.
Bake for 18-20 minutes. Let cool completely, then refrigerate for at least 3 hours, or overnight.
Cut into 12 bars, sprinkle powdered sugar on top (optional) and enjoy!