Preheat the oven to 375. Prepare a 9-13″ baking pan with parchment paper or spray with cooking spray.

Mix 1 teaspoon lemon zest into the granulated sugar and massage the zest and sugar so that the oils are released. This will give an extra burst of lemon flavor.

In the bowl of a stand mixer fitted with a paddle attachment or a food processor, stir together the flour, sugar, baking powder and salt.

Add the room temperature butter and pulse the food processor or mix with the mixer until the mixture resembles fine crumbs. You can also use a pastry cutter. Add the large whole egg and mix until incorporated.

Pour 2/3rds of the crumble mixture into your prepared crust and press into the bottom of the pan. Bake at 375 for 10-12 minutes or until the crust is starting to brown. Remove from oven and pour over the filling.

Filling

In a separate bowl mix together the sweetened condensed milk, lemon zest, lemon juice, vanilla extract and egg yolks. Pour over the pre-baked crust and spread evenly.

Crumble the remaining crumb mixture over the top, of the lemon filling,  you can squeeze the mixture together to make larger crumbs or just sprinkle the top with the crumbly mixture. Return the pan to the oven and bake at 375 F for an additional 20-25 minutes, the top with be golden brown and the filling set.

Remove the pan from the oven, place on a wire rack and let cool to room temperature. Chill for a few hours or over night, the cut and serve.