Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan and line it with parchment paper for easy removal. In a mixing bowl, cream together 1 cup of softened unsalted butter and ½ cup of superfine sugar until the mixture is light and fluffy. Add in 1 teaspoon of pure vanilla extract, then combine 2 cups of all-purpose flour and a pinch of salt in another bowl. Gradually add the dry ingredients to the butter mixture, mixing just until incorporated. Press about two-thirds of the dough evenly into the prepared pan and refrigerate it for 15 minutes.

While the crust is chilling, bake it in the preheated oven for 15 minutes or until it's lightly golden brown around the edges. Once baked, remove the crust from the oven and allow it to cool slightly while you prepare the frangipane filling.

In a mixing bowl, beat together ½ cup of softened butter, 1 teaspoon each of pure vanilla and almond extracts, and ¾ cup of sugar until creamy and smooth. Crack in 2 room temperature eggs one at a time, mixing well after each addition. Gradually stir in 1 cup of almond flour and ¼ cup of all-purpose flour (or gluten-free flour) to create a thick, creamy frangipane filling.

Spread the almond frangipane evenly over the cooled shortbread crust. Crumble the remaining shortbread dough on top of the filling. Sprinkle with sliced almonds. Return the baking pan to the oven and bake for an additional 35-37 minutes.

Once baked, remove the Almond Croissant Cookie Bars from the oven and let them cool in the pan for 10-15 minutes. Transfer to a wire rack and allow them to cool completely. Dust with powdered sugar before slicing into squares.