To Make the Shortbread Crust:

Preheat the oven to 350°F (180°C). Grease an 8 x 8-inch (20 x 20-cm) pan with butter or nonstick cooking spray and then line the pan with parchment paper, leaving an overhang on all sides. Grease the parchment with butter or nonstick cooking spray and then set it aside.

In a medium bowl, whisk together the granulated sugar, light brown sugar, flour, and salt. Slowly add the melted butter and vanilla to the flour mixture, stirring with a fork until combined. Press the mixture into the bottom of the prepared pan.

Bake for 15 minutes, until puffed and lightly golden.

To Make the Filling:

In a large bowl, whisk together the eggs and granulated sugar until well combined and thick. Add the flour, lemon juice, and lemon zest; whisk until combined, then pour it onto the warm crust.

Bake for 25 minutes, until the center is just set. Remove the bars from the oven and allow them to cool completely in the pan.

Once cool, run a knife around the edges of the pan to remove the lemon bars and then slice them into squares.

To Make the Brûléed Topping:

Just before serving, generously sprinkle each lemon bar square with granulated sugar. Use a kitchen torch to caramelize the sugar until it is amber or golden brown.

Allow the bars to rest a few minutes so the sugar can harden and crisp before serving. Cover and store leftover lemon bars in an airtight container in the refrigerator for up to 5 days.