Step 1: Preheat the Oven

Preheat your oven to 350°F (175°C). Line a 9×9-inch square baking pan with parchment paper or lightly grease it to prevent sticking.

Step 2: Prepare the Shortbread Crust

In a medium bowl, whisk together the flour, powdered sugar, and salt. Add the cold, cubed butter. Using a pastry cutter or your fingers, work the butter into the dry ingredients until it forms a crumbly mixture. Press the dough into the bottom of your prepared pan, creating an even layer. Bake in the preheated oven for 10-12 minutes or until the crust is lightly golden. Once done, remove from the oven and let it cool slightly.

Step 3: Make the Butter Tart Filling

In a large mixing bowl, whisk together the melted butter, brown sugar, vanilla extract, and flour until smooth. Add the eggs one at a time, mixing after each addition, until the mixture is well-combined. Stir in the salt and chopped nuts (if using).

Step 4: Assemble the Squares

Pour the butter tart filling over the cooled shortbread crust. Spread it evenly with a spatula. Bake the entire pan for 30-35 minutes or until the filling is set and the top is golden brown. A toothpick inserted into the center should come out clean when the squares are done.

Step 5: Cool and Cut

Allow the butter tart squares to cool completely in the pan. Once cooled, lift them out using the parchment paper (if used) and cut into squares. Serve and enjoy!