Preheat the oven to 350 degrees. Grease and 8x8 square baking pan and line with parchment paper all the way up the sides. Press it up the sides to make sure it is stuck to the pan as good as possible.
Combine the butter, sugar, and flour for the crust in a large bowl or the bowl of your stand mixer. Using the paddle attachment, or a spatula, mix until well-combined and the mixture comes together. Take out ⅓ cup shortbread and set aside to crumble on top of the bars. Press the rest of the crust into the bottom of the prepared pan. Bake for 20 minutes. Cool on a cooling rack while you make the filling.
Mix all the filling ingredients together and pour over the slightly cooled crust. Crumble the ⅓ cup of the shortbread over the top of the filling. Continue baking for 40 minutes until set and slightly golden on top.
Cool completely before cutting and serving.